in which the trivial-casts word to the wise is tucked into a help note

in which the trivial-casts word to the wise is tucked into a help note

The top level message shouldn't be too long; the
replaced-by-coercion/temporary-variable advice can live in a note. Also,
don't mention type ascription when it's not actually available as a real
thing. (The current state of discussion on the type ascription tracking
issue #23416 makes one rather suspect it will never be a stable thing in
its current form, but that's not for us to adjudicate in this commit.)

While we're here, yank out the differentiating parts of the
numeric/other conditional and only have one codepath emitting the
diagnostic.