Add a note that users of FATAL warnings are at risk during upgrades

Apparently it isn't obvious to all of our users that use of FATAL
warnings means that peoples code may break when they upgrade due
to the introduction of new warnings. Accordingly we document that
the use of FATAL warnings is entirely at the users risk, and that
we do not and will not consider the introduction of a warning as
an incompatible change, and that in particular the use of features
which are discouraged, unspecified, or whatnot may in the future
trigger new warnings, and thus under FATAL warnings break their code.