Merge branch 'add-authorized-key-commend-note' into 'master'

Add AuthorizedKeysCommand note for installations from source

See merge request gitlab-org/gitlab!31424

doc/administration/operations/fast_ssh_key_lookup.md

@@ -73,6 +73,12 @@ new one, and attempting to pull a repository.
 NOTE: **Note:** For Omnibus Docker, `AuthorizedKeysCommand` is setup by default in
 GitLab 11.11 and later.
 
+NOTE: **Note:** For Installations from source, the command would be located at
+`/home/git/gitlab-shell/bin/gitlab-shell-authorized-keys-check` if [the install from source](../../install/installation.md#install-gitlab-shell) instructions were followed.
+You might want to consider creating a wrapper script somewhere else since this command needs to be
+owned by `root` and not be writable by group or others. You could also consider changing the ownership of this command
+as required, but that might require temporary ownership changes during `gitlab-shell` upgrades.
+
 CAUTION: **Caution:** Do not disable writes until SSH is confirmed to be working
 perfectly, because the file will quickly become out-of-date.