docs: Clarify that signal functions have no feedback.

docs: Clarify that signal functions have no feedback.

Bug #16652 complains that pg_reload_conf() returned true, even though
the configuration file contained errors. That's the way pg_reload_conf()
works, by design, but the documentation wasn't very clear on it. Clarify
that a 'true' return value only means that the signal was sent
successfully. Also add links to the system views that can be used to
check the configuration files for errors.

David G. Johnston, with some rewording by me.

Discussion: https://www.postgresql.org/message-id/CAKFQuwax6GxhUQEes0D045UtXG-fBraM39_6UMd5JyR5K1HWCQ%40mail.gmail.com