cmd/compile: remove references to go.y and update documentation

This is a comment/documentation change only but for a minor
code change in the file and package_ methods (move recognition
of semi to match grammar better).

Per request from r.

Change-Id: I81ec985cc5831074d9eb5e8ffbf7e59466284819
Reviewed-on: https://go-review.googlesource.com/17202Reviewed-by: Rob Pike <r@golang.org>