cmd/doc: format field doc comments when printing entire struct

cmd/doc: format field doc comments when printing entire struct

cmd/doc passes structs to go/format, but that means that comments
on fields within those structs don't look like what cmd/doc prints
when asked for a struct field directly. Tweak the field comments
so that they look the same either way.

Fixes #56592

Change-Id: I198cb7a58e3d8558406c386072c630332f91c6b0
Reviewed-on: https://go-review.googlesource.com/c/go/+/483055
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Ian Lance Taylor <iant@golang.org>