Document gitaly timeouts

This documents the gitaly timeouts that can be configured in our admin
panel.

doc/user/admin_area/settings/gitaly_timeouts.md

+---
+type: reference
+---
+
+# Gitaly timeouts
+
+![gitaly timeouts](img/gitaly_timeouts.png)
+
+3 timeout types can be configured to make sure that long running
+Gitaly calls don't needlessly take up resources.
+
+- Default timeout
+
+This timeout is the default for most Gitaly calls.
+It should be shorter than the worker timeout that can be configured
+for
+[Puma](https://docs.gitlab.com/omnibus/settings/puma.html#puma-settings)
+or [Unicorn](https://docs.gitlab.com/omnibus/settings/unicorn.html).
+This makes sure that Gitaly calls made within a web request cannot
+exceed these the entire request timeout.
+
+The default for this timeout is 55 seconds.
+
+- Fast timeout
+
+This is the timeout for very short Gitaly calls.
+
+The default for this timeout is 10 seconds.
+
+- Medium timeout
+
+This timeout should be between the default and the fast timeout
+
+The default for this timeout is 30 seconds.

doc/user/admin_area/settings/index.md

@@ -24,6 +24,7 @@ include:
 - [Protected paths](protected_paths.md) **(CORE ONLY)**
 - [Help messages for the `/help` page and the login page](help_page.md)
 - [Push event activities limit and bulk push events](push_event_activities_limit.md)
+- [Gitaly timeouts](gitaly_timeouts.md)
 
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