[racket] proposed minor docfix for parser-tools/lex ?

Based on my experiments, it appears that parser-tools/lex picks the rule with the longest match, breaking ties in favor of the rule appearing first. As far as I can tell, this fact is not documented in the lexer manual. It says:

"Produces a function that takes an input-port, matches the re’s against the buffer, and returns the result of executing the corresponding action-expr."

Here's the text of my proposed commit. Unless I see a comment to the contrary, I'll commit this later today or tomorrow.

Thanks!

John