Clarify git housekeeping

doc/administration/housekeeping.md

@@ -12,13 +12,24 @@ to turn it off, go to **Admin area > Settings**
 
 ## Manual housekeeping
 
-The housekeeping function runs `git gc` ([man page][man]) on the current
-project Git repository.
+The housekeeping function will run a `repack` or `gc` depending on the
+"Automatic Git repository housekeeping" settings configured in **Admin area > Settings**
 
-`git gc` runs a number of housekeeping tasks, such as compressing file
-revisions (to reduce disk space and increase performance) and removing
-unreachable objects which may have been created from prior invocations of
-`git add`.
+For example in the following scenario a `git repack -d` will be executed:
+
++ Project: pushes since gc counter (`pushes_since_gc`) = `10`
++ Git GC period = `200`
++ Full repack period = `50`
+
+When the `pushes_since_gc` value is 50 a `repack -A -d --pack-kept-objects` will run, similarly when
+the `pushes_since_gc` value is 200 a `git gc` will be run.
+
++ `git gc` ([man page][man-gc]) runs a number of housekeeping tasks,
+such as compressing filerevisions (to reduce disk space and increase performance)
+and removing unreachable objects which may have been created from prior invocations of
+`git add`. 
+
++ `git repack` ([man page][man-repack]) re-organize existing packs into a single, more efficient pack.
 
 You can find this option under your **[Project] > Edit Project**.
 
@@ -27,4 +38,5 @@ You can find this option under your **[Project] > Edit Project**.
 ![Housekeeping settings](img/housekeeping_settings.png)
 
 [ce-2371]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2371 "Housekeeping merge request"
-[man]: https://www.kernel.org/pub/software/scm/git/docs/git-gc.html "git gc man page"
+[man-gc]: https://www.kernel.org/pub/software/scm/git/docs/git-gc.html "git gc man page"
+[man-repack]: https://www.kernel.org/pub/software/scm/git/docs/git-repack.html
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