Clarify referential actions in docs of CREATE/ALTER TABLE

Clarify referential actions in docs of CREATE/ALTER TABLE

The documentation of ON DELETE and ON UPDATE uses the term "action",
which is also used on the ALTER TABLE page for other purposes.  This
commit renames the term to "referential_action", which is more
consistent with the SQL specification.  The new term is now used on the
documentation of both CREATE TABLE and ALTER TABLE for consistency.

Reported-by: Brigitte Blanc-Lafay
Author: Lætitia Avrot
Reviewed-by: Álvaro Herrera
Discussion: https://postgr.es/m/CAB_COdiHEVVs0uB+uYCjjYUwQ4YFFekppq+Xqv6qAM8+cd42gA@mail.gmail.com