pgsql: Document new trigger-related forms of ALTER FOREIGN TABLE.

Document new trigger-related forms of ALTER FOREIGN TABLE.

Oversight in commit 7cbe57c34dec4860243e6d0f81738cfbb6e5d069.
Back-patch to 9.4, where that commit first appeared.  In passing,
release-note the FDW API postcondition change from the same commit.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/10718612fb23a5853e56eb792a6b1503ebddf190

Modified Files
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doc/src/sgml/ref/alter_foreign_table.sgml |   46 +++++++++++++++++++++++++++++
doc/src/sgml/release-9.4.sgml             |   12 ++++++++
2 files changed, 58 insertions(+)