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
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master

Details
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http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=9f2d7f6e2de23ecc1cfdd4172e23bd8b949fbcd2

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