manpage: clarify remote-shell daemon user@ handling

manpage: clarify remote-shell daemon user@ handling

The description of user@host::module transfers over a remote shell only
documented the "ssh -l ssh-user" form, which led readers to conclude that
user@ never reaches the remote shell. In fact, for the simple
`--rsh=ssh user@host::module` form the user@ prefix is used both as the
ssh login user (ssh -l user) and as the rsync-user offered to the module;
the two are the same name. rsync only omits its own -l when the remote
shell command already specifies one, in which case user@ becomes the
rsync-user alone.

Spell out the default behaviour and why the explicit -l is needed to use
a different ssh login than the rsync-user.