Improve documentation about array concat operator vs. underlying functions.

The documentation implied that there was seldom any reason to use the
array_append, array_prepend, and array_cat functions directly.  But that's
not really true, because they can help make it clear which case is meant,
which the || operator can't do since it's overloaded to represent all three
cases.  Add some discussion and examples illustrating the potentially
confusing behavior that can ensue if the parser misinterprets what was
meant.

Per a complaint from Michael Herold.  Back-patch to 9.2, which is where ||
started to behave this way.