Whitespace recursion warning (issue #2)

[wconrad]

This patch adds to the documentation a warning about the unintended consequences of "whitespace recursion."

[jarhart]

Heh, I never even considered that. Good catch.

I would recommend defining whitespace rules as fragments or after the close of a %whitespace block, rather than before the %whitespace directive. My thinking is that whitespace is usually the least interesting part of a language and therefore should be at the bottom of the grammar.