Update ganeti-rapi man page for new user option

Instead of duplicating what is already described in the full
documentation, a reference is added instead. Man pages can't use
constants or assertions, therefore it is easier for them to get out of
sync with the code.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>