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
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/f333d35428c1cba8d35065b6dbb2dd46e18bd929

Modified Files
--------------
doc/src/sgml/ref/alter_table.sgml | 21 +++++++++++++--------
1 file changed, 13 insertions(+), 8 deletions(-)