[dev.regabi] cmd/compile: clean up and document formatting

[dev.regabi] cmd/compile: clean up and document formatting

Some cleanup left over from moving the Type and Sym formatting to types.
And then document what the type formats are, now that it's clear.

Passes buildall w/ toolstash -cmp.

Change-Id: I35cb8978f1627db1056cb8ab343ce6ba6c99afad
Reviewed-on: https://go-review.googlesource.com/c/go/+/275780
Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>