documentation note

[Jim Wilson]

It is generally true that you need to use the most recent version of gcc
with the most recent version of binutils.  Both gcc and binutils are
evolving, and they are always (in theory) compatible with each other as
they evolve, but we can't ensure that the most recent version of gcc will
work with all binutils releases, or that the most recent version of binutils
will work with all gcc releases.

Jim