pgsql: Doc: clarify locking requirements for ALTER TABLE ADD FOREIGN KE

Doc: clarify locking requirements for ALTER TABLE ADD FOREIGN KEY.

The docs explained that a SHARE ROW EXCLUSIVE lock is needed on the
referenced table, but failed to say the same about the table being
altered.  Since the page says that ACCESS EXCLUSIVE lock is taken
unless otherwise stated, this left readers with the wrong conclusion.

Discussion: https://postgr.es/m/834603375.3470346.1586482852542@mail.yahoo.com

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/1d676ffd41be443d0c7a715f3e03841430ba4a2b

Modified Files
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doc/src/sgml/ref/alter_table.sgml | 21 ++++++++++++++++-----
1 file changed, 16 insertions(+), 5 deletions(-)