Copyedit plugins docs

[ci skip]

doc/administration/index.md

@@ -46,6 +46,7 @@ Learn how to install, configure, update, and maintain your GitLab instance.
 - [Uploads configuration](uploads.md): Configure GitLab uploads storage.
 [source installations](../install/installation.md#installation-from-source).
 - [Environment variables](environment_variables.md): Supported environment variables that can be used to override their defaults values in order to configure GitLab.
+- [Plugins](plugins.md): With custom plugins, GitLab administrators can introduce custom integrations without modifying GitLab's source code.
 - **(Starter/Premium)** [Elasticsearch](../integration/elasticsearch.md): Enable Elasticsearch to empower GitLab's Advanced Global Search. Useful when you deal with a huge amount of data.
 - **(Premium)** [External Classification Policy Authorization](../user/admin_area/settings/external_authorization.md)
 

doc/administration/plugins.md

-# Plugins
+# GitLab Plugin system
 
-**Note:** Plugins must be configured on the filesystem of the GitLab
-server. Only GitLab server administrators will be able to complete these tasks.
-Please explore [system hooks] or [webhooks] as an option if you do not
-have filesystem access. 
+> Introduced in GitLab 10.6.
 
-Introduced in GitLab 10.6.
+With custom plugins, GitLab administrators can introduce custom integrations
+without modifying GitLab's source code.
 
-A plugin will run on each event so it's up to you to filter events or projects within a plugin code. You can have as many plugins as you want. Each plugin will be triggered by GitLab asynchronously in case of an event. For a list of events please see [system hooks] documentation.
+NOTE: **Note:**
+Instead of writing and supporting your own plugin you can make changes
+directly to the GitLab source code and contribute back upstream. This way we can
+ensure functionality is preserved across versions and covered by tests.
+
+NOTE: **Note:**
+Plugins must be configured on the filesystem of the GitLab server. Only GitLab
+server administrators will be able to complete these tasks. Explore
+[system hooks] or [webhooks] as an option if you do not have filesystem access.
+
+A plugin will run on each event so it's up to you to filter events or projects
+within a plugin code. You can have as many plugins as you want. Each plugin will
+be triggered by GitLab asynchronously in case of an event. For a list of events
+see the [system hooks] documentation.
 
 ## Setup
 
-Plugins must be placed directly into `plugins` directory, subdirectories will be ignored. 
-There is an `example` directory inside `plugins` where you can find some basic examples. 
+The plugins must be placed directly into the `plugins` directory, subdirectories
+will be ignored. There is an
+[`example` directory inside `plugins`](https://gitlab.com/gitlab-org/gitlab-ce/tree/master/plugins/examples)
+where you can find some basic examples.
 
 Follow the steps below to set up a custom hook:
 
-1. On the GitLab server, navigate to the project's plugin directory.
+1. On the GitLab server, navigate to the plugin directory.
    For an installation from source the path is usually
    `/home/git/gitlab/plugins/`. For Omnibus installs the path is
    usually `/opt/gitlab/embedded/service/gitlab-rails/plugins`.
-1. Inside the `plugins` directory, create a file with a name of your choice, but without spaces or special characters.
+1. Inside the `plugins` directory, create a file with a name of your choice,
+   without spaces or special characters.
 1. Make the hook file executable and make sure it's owned by the git user.
-1. Write the code to make the plugin function as expected. Plugin can be
-   in any language. Ensure the 'shebang' at the top properly reflects the language
-   type. For example, if the script is in Ruby the shebang will probably be
-   `#!/usr/bin/env ruby`.
-1. The data to the plugin will be provided as JSON on STDIN. It will be exactly same as one for [system hooks]
+1. Write the code to make the plugin function as expected. That can be
+   in any language, and ensure the 'shebang' at the top properly reflects the
+   language type. For example, if the script is in Ruby the shebang will
+   probably be `#!/usr/bin/env ruby`.
+1. The data to the plugin will be provided as JSON on STDIN. It will be exactly
+   same as for [system hooks]
 
-That's it! Assuming the plugin code is properly implemented the hook will fire
-as appropriate. Plugins file list is updated for each event. There is no need to restart GitLab to apply a new plugin.
+That's it! Assuming the plugin code is properly implemented, the hook will fire
+as appropriate. The plugins file list is updated for each event, there is no
+need to restart GitLab to apply a new plugin.
 
 If a plugin executes with non-zero exit code or GitLab fails to execute it, a
 message will be logged to `plugin.log`.
 
 ## Validation
 
-Writing own plugin can be tricky and its easier if you can check it without altering the system. 
-We provided a rake task you can use with staging environment to test your plugin before using it in production. 
-The rake task will use a sample data and execute each of plugins. By output you should be able to determine if 
-system sees your plugin and if it was executed without errors.
+Writing your own plugin can be tricky and it's easier if you can check it
+without altering the system. A rake task is provided so that you can use it
+in a staging environment to test your plugin before using it in production.
+The rake task will use a sample data and execute each of plugin. The output
+should be enough to determine if the system sees your plugin and if it was
+executed without errors.
 
 ```bash
 # Omnibus installations
 sudo gitlab-rake plugins:validate
 
 # Installations from source
+cd /home/git/gitlab
 bundle exec rake plugins:validate RAILS_ENV=production
 ```
 
-Example of output can be next: 
+Example of output:
 
 ```
--> bundle exec rake plugins:validate RAILS_ENV=production
 Validating plugins from /plugins directory
 * /home/git/gitlab/plugins/save_to_file.clj succeed (zero exit code)
 * /home/git/gitlab/plugins/save_to_file.rb failure (non-zero exit code)
 ```
 
-[hooks]: https://git-scm.com/book/en/v2/Customizing-Git-Git-Hooks#Server-Side-Hooks
 [system hooks]: ../system_hooks/system_hooks.md
 [webhooks]: ../user/project/integrations/webhooks.md
-[5073]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5073
-[93]: https://gitlab.com/gitlab-org/gitlab-shell/merge_requests/93
-