Clean up environment variables doc

[ci skip]

doc/administration/environment_variables.md

 # Environment Variables
 
-## Introduction
+GitLab exposes certain environment variables which can be used to override
+their defaults values.
 
-Commonly people configure GitLab via the `gitlab.rb` configuration file in the Omnibus package.
+People usually configure GitLab via `/etc/gitlab/gitlab.rb` for Omnibus
+installations, or `gitlab.yml` for installations from source.
 
-But if you prefer to use environment variables we allow that too.
+Below you will find the supported environment variables which you can use to
+override certain values.
 
 ## Supported environment variables
 
-Variable | Type | Explanation
+Variable | Type | Description
 -------- | ---- | -----------
 `GITLAB_ROOT_PASSWORD`      | string | Sets the password for the `root` user on installation
-`GITLAB_HOST` | url | Hostname of the GitLab server includes http or https
-`RAILS_ENV` | production / development / staging / test | Rails environment
-`DATABASE_URL` | url | For example: postgresql://localhost/blog_development
-`GITLAB_EMAIL_FROM` | email | Email address used in the "From" field in mails sent by GitLab
-`GITLAB_EMAIL_DISPLAY_NAME` | string | Name used in the "From" field in mails sent by GitLab
-`GITLAB_EMAIL_REPLY_TO` | email | Email address used in the "Reply-To" field in mails sent by GitLab
-`GITLAB_UNICORN_MEMORY_MIN` | integer | The minimum memory threshold (in bytes) for the Unicorn worker killer
-`GITLAB_UNICORN_MEMORY_MAX` | integer | The maximum memory threshold (in bytes) for the Unicorn worker killer
+`GITLAB_HOST`               | string | The full URL of the GitLab server (including `http://` or `https://`)
+`RAILS_ENV`                 | string | The Rails environment; can be one of `production`, `development`, `staging` or `test`
+`DATABASE_URL`              | string | The database URL; is of the form: `postgresql://localhost/blog_development`
+`GITLAB_EMAIL_FROM`         | string | The e-mail address used in the "From" field in e-mails sent by GitLab
+`GITLAB_EMAIL_DISPLAY_NAME` | string | The name used in the "From" field in e-mails sent by GitLab
+`GITLAB_EMAIL_REPLY_TO`     | string | The e-mail address used in the "Reply-To" field in e-mails sent by GitLab
+`GITLAB_UNICORN_MEMORY_MIN` | integer | The minimum memory threshold (in bytes) for the Unicorn worker killer (supported only in Omnibus)
+`GITLAB_UNICORN_MEMORY_MAX` | integer | The maximum memory threshold (in bytes) for the Unicorn worker killer (supported only in Omnibus)
 
 ## Complete database variables
 
 The recommended way of specifying your database connection information is to set
 the `DATABASE_URL` environment variable. This variable only holds connection
-information (adapter, database, username, password, host and port), but not
-behavior information (encoding, pool). If you don't want to use `DATABASE_URL`
-and/or want to set database behavior information, you will have to:
+information (`adapter`, `database`, `username`, `password`, `host` and `port`),
+but not behavior information (`encoding`, `pool`). If you don't want to use
+`DATABASE_URL` and/or want to set database behavior information, you will have
+to either:
 
-- copy our template `config/database.yml` file: `cp config/database.yml.env config/database.yml`
+- copy our template file: `cp config/database.yml.env config/database.yml`, or
 - set a value for some `GITLAB_DATABASE_XXX` variables
 
-The list of `GITLAB_DATABASE_XXX` variables that you can set is as follow:
+The list of `GITLAB_DATABASE_XXX` variables that you can set is:
 
 Variable | Default value | Overridden by `DATABASE_URL`?
---- | --- | ---
-`GITLAB_DATABASE_ADAPTER` | `postgresql` | Yes
+-------- | ------------- | -----------------------------
+`GITLAB_DATABASE_ADAPTER`   | `postgresql` (for MySQL use `mysql2`) | Yes
 `GITLAB_DATABASE_DATABASE`  | `gitlab_#{ENV['RAILS_ENV']`           | Yes
 `GITLAB_DATABASE_USERNAME`  | `root`                                | Yes
 `GITLAB_DATABASE_PASSWORD`  | None                                  | Yes
@@ -47,10 +51,11 @@ Variable | Default value | Overridden by `DATABASE_URL`?
 ## Adding more variables
 
 We welcome merge requests to make more settings configurable via variables.
-Please make changes in the `config/initializers/1_settings.rb` file.
-Please stick to the naming scheme `GITLAB_#{name in 1_settings.rb in upper case}`.
+Please make changes in the `config/initializers/1_settings.rb` file and stick
+to the naming scheme `GITLAB_#{name in 1_settings.rb in upper case}`.
 
 ## Omnibus configuration
 
-It's possible to preconfigure the GitLab image by adding the environment variable: `GITLAB_OMNIBUS_CONFIG` to docker run command.
+It's possible to preconfigure the GitLab docker image by adding the environment
+variable `GITLAB_OMNIBUS_CONFIG` to the `docker run` command.
 For more information see the ['preconfigure-docker-container' section in the Omnibus documentation](http://doc.gitlab.com/omnibus/docker/#preconfigure-docker-container).