cmd/godoc: deprecate and point to cmd/pkgsite

Fixes golang/go#49212.

Change-Id: I335749bfecc60f582968af601d9fa030f038e2be
Reviewed-on: https://go-review.googlesource.com/c/tools/+/349051
Trust: Jonathan Amsterdam <jba@google.com>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

cmd/godoc/doc.go

@@ -108,5 +108,7 @@ see https://golang.org/pkg/testing/#hdr-Examples for the conventions.
 See "Godoc: documenting Go code" for how to write good comments for godoc:
 https://golang.org/doc/articles/godoc_documenting_go_code.html
 
+// Deprecated: godoc cannot select what version of a package is displayed.
+// Instead, use golang.org/x/pkgsite/cmd/pkgsite.
 */
 package main // import "golang.org/x/tools/cmd/godoc"