The patch rewrites the content of doc.go file. The file describes some
general rules of the mapping between Go assembly syntax and GNU syntax.
And it gives some Go assembly examples and corresponding GNU assembly
examples.

The patch changes the doc.go to use standard doc comment format so that
the link https://golang.org/cmd/internal/obj/arm64/ can display it.

Assembly document framework is mainly contributed by Eric Fang <Eric.Fang@arm.com>
Documentation work is contributed by Eric Fang and Fannie Zhang <Fannie.Zhang@arm.com>

Change-Id: I8b3f6d6c6b91afdc2c44602e8f796beea905085e
Reviewed-on: https://go-review.googlesource.com/102055Reviewed-by: Rob Pike <r@golang.org>