Update Patch docs to include gitaly

ZD: https://gitlab.zendesk.com/agent/tickets/82082

Quoted:

while updating our GitLab instance, I noticed that the 'Universal update guide for patch versions'[0] seems to be missing the instructions to update Gitaly.

Another improvement we'd like to see would be to make all the paths relative, i.e. it often refers to /home/git/..., but our installation is under a different path. But since the different projects will still be under the same parent, using relative paths would make the easily copy-pastable for both us and people using the default /home/git :-)