go/parser: more precise comment
go/parser: more precise comment

See also CL 7383051 for details.

R=adonovan, bradfitz
CC=golang-dev
https://codereview.appspot.com/7378063

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