doc/gitaly: Adapt configuration to recommend setting runtime dir

Adapt the documentation for source installations to not recommend
setting the internal socket directory anymore. Instead, users should
configure the new runtime directory, which besides other files and
directories also holds internal sockets now.

doc/administration/gitaly/configure_gitaly.md

@@ -300,7 +300,7 @@ disable enforcement. For more information, see the documentation on configuring
    ```toml
    listen_addr = '0.0.0.0:8075'
 
-   internal_socket_dir = '/var/opt/gitlab/gitaly'
+   runtime_dir = '/var/opt/gitlab/gitaly'
 
    [logging]
    format = 'json'
@@ -308,6 +308,9 @@ disable enforcement. For more information, see the documentation on configuring
    dir = '/var/log/gitaly'
    ```
 
+    For GitLab 14.9 and earlier, set `internal_socket_dir = '/var/opt/gitlab/gitaly'` instead
+    of `runtime_dir`.
+
 1. Append the following to `/home/git/gitaly/config.toml` for each respective Gitaly server:
 
    On `gitaly1.internal`: