pgsql: Complete the documentation of the USAGE privilege for foreign se

Complete the documentation of the USAGE privilege for foreign servers

The GRANT reference page failed to mention that the USAGE privilege
allows modifying associated user mappings, although this was already
documented on the CREATE/ALTER/DROP USER MAPPING pages.

Branch
------
REL9_0_STABLE

Details
-------
http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=071f810c982be5b3789ff408b51e2777ae32ebdd

Modified Files
--------------
doc/src/sgml/ref/grant.sgml |    6 ++++--
1 files changed, 4 insertions(+), 2 deletions(-)