[PATCH] doc: Clarify pg_checksums --filenode mode restriction

Hi,

The pg_checksums reference page describes --filenode without stating that
it is only available in check mode. The command rejects it with --enable
or --disable, so a reader cannot use this option to limit those operations
to a single relation.

The attached PostgreSQL documentation patch adds that restriction to the
option description. It is against master at c11eb5de01b and is intended
for review and application. It changes no program behavior and has no
performance or platform-specific effects.

Validation: git diff --check, XML well-formedness of the changed reference
page, and DocBook XML 4.5 DTD validation of the changed paragraph passed.
The page contains no tabs or non-breaking spaces. The existing tests in
src/bin/pg_checksums/t/002_actions.pl cover both rejected combinations.
Those TAP tests and the full build/documentation build were not run.

This patch and submission note were prepared with OpenAI Codex assistance.

Thanks,
Sri Vishnu