pgsql: Doc: fix missing explanation of default object privileges.

Doc: fix missing explanation of default object privileges.

The GRANT reference page, which lists the default privileges for new
objects, failed to mention that USAGE is granted by default for data
types and domains. As a lesser sin, it also did not specify anything
about the initial privileges for sequences, FDWs, foreign servers,
or large objects. Fix that, and add a comment to acldefault() in the
probably vain hope of getting people to maintain this list in future.

Noted by Laurenz Albe, though I editorialized on the wording a bit.
Back-patch to all supported branches, since they all have this behavior.

Discussion: https://postgr.es/m/1507620895.4152.1.camel@cybertec.at

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/fbac00a4a8d3d9ff97ceb7176fce038fea5239b5

Modified Files
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doc/src/sgml/ref/grant.sgml | 20 +++++++++++++++-----
src/backend/utils/adt/acl.c | 4 +++-
2 files changed, 18 insertions(+), 6 deletions(-)