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>

man/ganeti-rapi.rst

@@ -34,28 +34,13 @@ in the same format as for the node and master daemon.
 ACCESS CONTROLS
 ---------------
 
-All query operations are allowed without authentication. Only the
+Most query operations are allowed without authentication. Only the
 modification operations require authentication, in the form of basic
 authentication.
 
 The users and their rights are defined in the
-``@LOCALSTATEDIR@/lib/ganeti/rapi/users`` file. The users
-should be listed one per line, in the following format::
-
-    username password options
-
-Currently the *options* field should equal the string ``write`` in
-order to actually give write permission for the given users. Example::
-
-    rclient   secret    write
-    guest     testpw
-
-The first user (*rclient*) has read-write rights, whereas the second
-user (*guest*) only has read (query) rights, and as such is no
-different than not using authentication at all.
-
-More details (including on how to use hashed passwords) can be found
-in the Ganeti documentation.
+``@LOCALSTATEDIR@/lib/ganeti/rapi/users`` file. Format of this file is
+described in the Ganeti documentation (``rapi.html``).
 
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