Project import rake task on Omnibus, plus errors

doc/development/import_project.md

@@ -47,7 +47,9 @@ This method will take longer to import than the other methods and will depend on
 
 ### Importing via a Rake task
 
-[`import.rake`](https://gitlab.com/gitlab-org/gitlab/blob/master/lib/tasks/gitlab/import_export/import.rake) was introduced for importing large GitLab project exports.
+> The [Rake task](https://gitlab.com/gitlab-org/gitlab/blob/master/lib/tasks/gitlab/import_export/import.rake) was [introduced](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/20724) in GitLab 12.6, replacing a GitLab.com Ruby script.
+
+This script was introduced in GitLab 12.6 for importing large GitLab project exports.
 
 As part of this script we also disable direct and background upload to avoid situations where a huge archive is being uploaded to GCS (while being inside a transaction, which can cause idle transaction timeouts).
 
@@ -63,9 +65,53 @@ Parameters:
 | `archive_path` | string | yes | Path to the exported project tarball you want to import |
 
 ```shell
-bundle exec rake "gitlab:import_export:import[root, root, testingprojectimport, /path/to/file.tar.gz]"
+bundle exec rake "gitlab:import_export:import[root, group/subgroup, testingprojectimport, /path/to/file.tar.gz]"
+```
+
+If you're running Omnibus, run the following Rake task:
+
+```shell
+gitlab-rake "gitlab:import_export:import[root, group/subgroup, testingprojectimport, /path/to/file.tar.gz]"
+```
+
+#### Troubleshooting
+
+Check the common errors listed below, what they mean, and how to fix them.
+
+##### `Exception: undefined method 'name' for nil:NilClass`
+
+The `username` is not valid.
+
+##### `Exception: undefined method 'full_path' for nil:NilClass`
+
+The `namespace_path` does not exist.
+For example, one of the groups or subgroups is mistyped or missing
+or you've specified the project name in the path.
+
+The task will only create the project.
+If you want to import it to a new group or subgroup then create it first.
+
+##### `Exception: No such file or directory @ rb_sysopen - (filename)`
+
+The specified project export file in `archive_path` is missing.
+
+##### `Name can contain only letters, digits, emojis ...`
+
+```plaintext
+Name can contain only letters, digits, emojis, '_', '.', dash, space. It must start with letter,
+digit, emoji or '_'. and Path can contain only letters, digits, '_', '-' and '.'. Cannot start
+with '-', end in '.git' or end in '.atom'
 ```
 
+The project name specified in `project_path` is not valid for one of the specified reasons.
+
+Only put the project name in `project_path`. If, for example, you put a path of subgroups in there
+it will fail with this error as `/` is not a valid character in a project name.
+
+##### `Name has already been taken and Path has already been taken`
+
+A project with that name already exists.
+
 ### Importing via the Rails console
 
 The last option is to import a project using a Rails console: