Not everyone is aware that go build is a wrapper for other
tools. Mention this in the text for go help build so people using
other build systems won't just wrap go build, which is usually a
mistake (it doesn't do incremental builds by default, for instance).

Update #11854.

Change-Id: I759f91f23ccd3671204c39feea12a3bfaf9f0114
Reviewed-on: https://go-review.googlesource.com/12625Reviewed-by: Ian Lance Taylor <iant@golang.org>