Change the default behavior when showing the package docs
for a command to elide the symbols. This makes

	go doc somecommand

show the top-level package docs only and hide the symbols,
which are probably irrelevant to the user. This has no effect
on explicit requests for internals, such as

	go doc somecommand.sometype

The new -cmd flag restores the old behavior.

Fixes #10733.

Change-Id: I4d363081fe7dabf76ec8e5315770ac3609592f80
Reviewed-on: https://go-review.googlesource.com/11953Reviewed-by: Russ Cox <rsc@golang.org>