pgsql: doc: Add referential actions to CREATE/ALTER TABLE synopsis

doc: Add referential actions to CREATE/ALTER TABLE synopsis

The general constraint synopsis references "referential_action", but
this was not further defined in the synopsis section.  Compared to the
level of detail that the synopsis gives to other subclauses, this
should surely be there.

extracted from a patch by Paul Martinez <hellopfm@gmail.com>

Discussion:
https://www.postgresql.org/message-id/flat/CACqFVBZQyMYJV=njbSMxf+rbDHpx=W=B7AEaMKn8dWn9OZJY7w@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/db9f287711ac49d9799f93f664d6d101ff8f5891

Modified Files
--------------
doc/src/sgml/ref/alter_table.sgml  | 4 ++++
doc/src/sgml/ref/create_table.sgml | 4 ++++
2 files changed, 8 insertions(+)