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>

src/cmd/doc/pkg.go

@@ -12,6 +12,7 @@ import (
 	"go/doc"
 	"go/format"
 	"go/parser"
+	"go/printer"
 	"go/token"
 	"io"
 	"log"
@@ -206,7 +207,17 @@ func (pkg *Package) newlines(n int) {
 // clears the stuff we don't want to print anyway. It's a bit of a magic trick.
 func (pkg *Package) emit(comment string, node ast.Node) {
 	if node != nil {
-		err := format.Node(&pkg.buf, pkg.fs, node)
+		var err error
+		if showSrc {
+			// Need an extra little dance to get internal comments to appear.
+			commentedNode := &printer.CommentedNode{
+				Node:     node,
+				Comments: pkg.file.Comments,
+			}
+			err = format.Node(&pkg.buf, pkg.fs, commentedNode)
+		} else {
+			err = format.Node(&pkg.buf, pkg.fs, node)
+		}
 		if err != nil {
 			log.Fatal(err)
 		}