doc: Clarify COPY FROM WHERE expression restrictions

doc: Clarify COPY FROM WHERE expression restrictions

Commit aa606b9316a disallowed generated columns in COPY FROM WHERE
expressions, and commit 21c69dc73f9 disallowed system columns.
However, the COPY reference page still mentions only the restriction
on subqueries.

Update the documentation to also list generated columns and system
columns as unsupported in COPY FROM WHERE expressions.

Backpatch the generated-column documentation change to all supported
versions. Backpatch the system-column documentation change to v19,
where that restriction was introduced.

Author: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Ayush Tiwari <ayushtiwari.slg01@gmail.com>
Discussion: https://postgr.es/m/CAHGQGwEgxErc54yVOAVWCsr1O=8pgw4oKRPuEQ9mfhkoYGR_XA@mail.gmail.com
Backpatch-through: 14