cmd/go: enable stringintconv and ifaceassert vet checks by default

cmd/go: enable stringintconv and ifaceassert vet checks by default

As per discussion on the accepted proposals, enable these vet checks by
default in the go command. Update corresponding documentation as well.

Updates #32479.
Updates #4483.

Change-Id: Ie93471930c24dbb9bcbf7da5deaf63bc1a97a14f
Reviewed-on: https://go-review.googlesource.com/c/go/+/232660
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>