Patch: update -fmessage-length docs

[Tom Tromey]

>>>>> "Geoff" == Geoffrey Keating <geoffk@apple.com> writes:

>> * doc/invoke.texi (Language Independent Options): Message length
>> defaults to 0.

Geoff> In the absence of evidence that the change was intentional, I assume
Geoff> that the docs are correct and the code is wrong, so this patch is not
Geoff> OK.

Thanks.  I filed a PR: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32614

Tom