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