Match pg_user_mappings limits to information_schema.user_mapping_options.

Both views replace the umoptions field with NULL when the user does not
meet qualifications to see it.  They used different qualifications, and
pg_user_mappings documented qualifications did not match its implemented
qualifications.  Make its documentation and implementation match those
of user_mapping_options.  One might argue for stronger qualifications,
but these have long, documented tenure.  pg_user_mappings has always
exhibited this problem, so back-patch to 9.2 (all supported versions).

Michael Paquier and Feike Steenbergen.  Reviewed by Jeff Janes.
Reported by Andrew Wheelwright.

Security: CVE-2017-7486

doc/src/sgml/catalogs.sgml

@@ -9990,8 +9990,11 @@ SELECT * FROM pg_locks pl LEFT JOIN pg_prepared_xacts ppx
       <entry></entry>
       <entry>
        User mapping specific options, as <quote>keyword=value</>
-       strings, if the current user is the owner of the foreign
-       server, else null
+       strings.  This column will show as null unless the current user
+       is the user being mapped, or the mapping is for
+       <literal>PUBLIC</literal> and the current user is the server
+       owner, or the current user is a superuser.  The intent is
+       to protect password information stored as user mapping option.
       </entry>
      </row>
     </tbody>