Update the documentation to clarify that SSH username can be different

What does this MR do and why?

Contributes to #432439 (closed)

Documentation states that one should use ssh -T git@gitlab.example.com to test the server connection. However, on gitlab.mpi-sws.org, the username is different -- it is git-rts. We just had a user who was quite confused that things didn't work despite following the docs since he tried git@gitlab.mpi-sws.org and of course that does not work...

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