go/doc: use go/doc/comment

go/doc: use go/doc/comment

[This CL is part of a sequence implementing the proposal #51082.
The design doc is at https://go.dev/s/godocfmt-design.]

Use go/doc/comment to implement the existing go/doc comment APIs,
as well as adding new APIs more tailored to the new world.

For #51082.

Change-Id: I05b97ecedbf7cf7b8dede7ace6736ed6d89204a9
Reviewed-on: https://go-review.googlesource.com/c/go/+/384265
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>