regexp/syntax: update documentation for Parse.
regexp/syntax: update documentation for Parse.

Syntax description was moved to the top level comment of this package.

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/11536043

apparentlymart/golang,tv42/old-go,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,apparentlymart/golang,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,tv42/old-go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme