Clarify INCLUDING COMMENTS behavior in CREATE TABLE and CREATE FOREIGN TABLE docs

Hi,

> Comments for the copied columns, constraints, and indexes will be copied.

The above current description of INCLUDING COMMENTS in the CREATE TABLE docs
appears inaccurate and incomplete. It says that comments on constraints are
copied, but in practice only comments on CHECK and NOT NULL constraints
are copied; comments on other constraints (such as primary keys) are not.
In addition, comments on extended statistics are copied, but this is not
documented.

The CREATE FOREIGN TABLE docs has a similar omission: comments on extended
statistics are also copied, but this is not mentioned.

Therefore I'd like to propose the attached patch to clarify the actual behavior
of INCLUDING COMMENTS. Thoughts?

Regards,

--
Fujii Masao