cmd/doc: add -all flag to print all documentation for package

Unlike the one for the old godoc, you need the -u flag to see
unexported symbols. This seems like the right behavior: it's
consistent.

For now at least, the argument must be a package, not a symbol.
This is also different from old godoc.

Required a little refactoring but also cleaned up a few things.

Update #25595

Leaving the bug open for now until we tackle
	go doc -all symbol

Change-Id: Ibc1975bfa592cb1e92513eb2e5e9e11e01a60095
Reviewed-on: https://go-review.googlesource.com/c/141977


Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>