doc: reference related commands from users(1) and groups(1)

* man/users.x: See also getent, who
* man/groups.x: See also getent
Addresses http://bugs.gnu.org/21735

man/groups.x

@@ -2,3 +2,5 @@
 groups \- print the groups a user is in
 [DESCRIPTION]
 .\" Add any additional description here
+[SEE ALSO]
+getent(1)

man/users.x

@@ -2,3 +2,5 @@
 users \- print the user names of users currently logged in to the current host
 [DESCRIPTION]
 .\" Add any additional description here
+[SEE ALSO]
+getent(1), who(1)