doc: Add WITHOUT OVERLAPS to ALTER TABLE reference docs

doc: Add WITHOUT OVERLAPS to ALTER TABLE reference docs

Commit fc0438b4e80 documented WITHOUT OVERLAPS in the CREATE TABLE
docs, but not in ALTER TABLE.  This commit adds the new syntax to the
ALTER TABLE docs.

Author: Paul A. Jungwirth <pj@illuminatedcomputing.com>
Discussion: https://www.postgresql.org/message-id/57ea0668-5205-426e-b934-efc89f2186c2@illuminatedcomputing.com