gssapi: document ZMQ_GSSAPI_PRINCIPAL as optional

Problem: the ZMQ_GSSAPI_PRINCIPAL socket option is described
as mandatory in the zmq_gssapi(7) manual page.  In fact it
is optional.

Solution: Describe ZMQ_GSSAPI_PRINCIPAL as optional.
If unspecified, default credentials are used.

doc/zmq_gssapi.txt

@@ -22,16 +22,17 @@ CLIENT AND SERVER ROLES
 -----------------------
 A socket using GSSAPI can be either client or server, but not both.
 
-To become either a GSSAPI client or server, the application sets the
-ZMQ_GSSAPI_PRINCIPAL option to provide the socket with the name of the principal
-for whom GSSAPI credentials should be acquired.
+To become a GSSAPI server, the application sets the ZMQ_GSSAPI_SERVER
+option on the socket.
 
-To become a GSSAPI server, the application additionally sets the
-ZMQ_GSSAPI_SERVER option on the socket.
+To become a GSSAPI client, the application sets the ZMQ_GSSAPI_SERVICE_PRINCIPAL
+option to the name of the principal on the server to which it intends to
+connect.
 
-To become a GSSAPI client, the application additionally sets the
-ZMQ_GSSAPI_SERVICE_PRINCIPAL option to the name of the principal of the server
-to which it intends to connect.
+On client or server, the application may additionally set the
+ZMQ_GSSAPI_PRINCIPAL option to provide the socket with the name of the
+principal for whom GSSAPI credentials should be acquired.  If this option
+is not set, default credentials are used.
 
 
 OPTIONAL ENCRYPTION