Warn against using plain git commands for housekeeping

doc/administration/housekeeping.md

@@ -62,6 +62,12 @@ Housekeeping also [removes unreferenced LFS files](../raketasks/cleanup.md#remov
 from your project on the same schedule as the `git gc` operation, freeing up storage space for your
 project.
 
+WARNING:
+Running `git gc` or `git repack` commands manually in the
+[repository folder](repository_storage_types.md#from-project-name-to-hashed-path)
+is discouraged. If the created pack files get incorrect access rights (that is, owned by the wrong user)
+browsing to the project page might result in `404` and `503` errors.
+
 ## How housekeeping handles pool repositories
 
 Housekeeping for pool repositories is handled differently from standard repositories. It is
@@ -76,7 +82,7 @@ This is the current call stack by which it is invoked:
 1. `ObjectPoolService#fetch`
 1. `Gitaly::FetchIntoObjectPoolRequest`
 
-To manually invoke it from a Rails console if needed, you can call
+To manually invoke it from a [Rails console](operations/rails_console.md) if needed, you can call
 `project.pool_repository.object_pool.fetch`. This is a potentially long-running task, though Gitaly
 times out in about 8 hours.
 

doc/administration/raketasks/check.md

@@ -204,12 +204,20 @@ See [LDAP Rake Tasks - LDAP Check](ldap.md#check) for details.
 The following are solutions to problems you might discover using the Rake tasks documented
 above.
 
-### Dangling commits
+### Dangling objects
 
-`gitlab:git:fsck` can find dangling commits. To fix them, try
-[enabling housekeeping](../housekeeping.md).
+The `gitlab:git:fsck` task can find dangling objects such as:
 
-If the issue persists, try triggering `gc` via the
+```plaintext
+dangling blob a12...
+dangling commit b34...
+dangling tag c56...
+dangling tree d78...
+```
+
+To delete them, try [running housekeeping](../housekeeping.md).
+
+If the issue persists, try triggering garbage collection via the
 [Rails Console](../operations/rails_console.md#starting-a-rails-console-session):
 
 ```ruby
@@ -217,6 +225,13 @@ p = Project.find_by_path("project-name")
 Repositories::HousekeepingService.new(p, :gc).execute
 ```
 
+If the dangling objects are younger than the 2 weeks default grace period,
+and you don't want to wait until they expire automatically, run:
+
+```ruby
+Repositories::HousekeepingService.new(p, :prune).execute
+```
+
 ### Delete references to missing remote uploads
 
 `gitlab-rake gitlab:uploads:check VERBOSE=1` detects remote objects that do not exist because they were