doc/man: remove docs about support for unix domain sockets

doc/man: remove docs about support for unix domain sockets

doc/man: support for unix domain sockets is not implemented, hence we
removed documentation about it.

(Note: the changes in this commit were the work of Rok Jaklič in
https://github.com/ceph/ceph/pull/48537. This pull request has been
raised because that pull request was for some mysterious reason causing
merge conflicts that were never resolved.)

Co-authored-by: Rok Jaklič rjaklic@gmail.com
Signed-off-by: Zac Dover <zac.dover@proton.me>