cmd/doc: make comments inside functions appear with -src

The old godoc didn't do this either, perhaps because it's a little
tricky, but it can be done using a special type from the go/printer
package. (Usually we just use go/format).

Fixes #28195.

Change-Id: Ic6d3df3953ba71128398ceaf9a133c798551b6b8
Reviewed-on: https://go-review.googlesource.com/c/143037


Reviewed-by: Robert Griesemer <gri@golang.org>