Trigraph warnings when compiling linux-2.4.0-prerelease1

[dewar@gnat.com]

<<Thanks for alerting us to this documentation bug.
>>

Boy, Joe, you really love the word bug don't you?

In fact the current documentation is clear and appears to match the
documentation. It is certainly reasonable to ask the question of
whether the warning should be effective even if trigraphs are enabled,
but running around shouting "BUG" is not the best policy here :-)