cmd/doc: add more convenient behavior for go doc -http with no args
If we're in a module, go to the module's page. Outside of a module, but
in a workspace go to the home page, and outside of a module or
workspace, show the stdlib docs.
For #68106
Change-Id: I911a90a0e2b0a2bbb622f56e32827d5bdfa7f2fd
Reviewed-on: https://go-review.googlesource.com/c/go/+/675235 Reviewed-by: Michael Matloob <matloob@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Alan Donovan <adonovan@google.com>