It's long-desired but was blocked by #26835. That is now fixed, so
it's easy. When -src is off, we behave as before. But with -src
set, initialize the go/doc package to preserve the original AST and
things flow very easily.

With -src, since you're seeing inside the package source anyway it
shows unexported fields and constants: you see the original source.
But you still need -u to ask about them.

Fixes #18807

Change-Id: I473e90323b4eff0735360274dc0d2d9dba16ff8b
Reviewed-on: https://go-review.googlesource.com/c/140959Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Andrew Gerrand <adg@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>