regexp: remove unnecessary sentence in doc comment.
regexp: remove unnecessary sentence in doc comment.

R=rsc, iant
CC=golang-codereviews
https://codereview.appspot.com/53190046

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang