cmd/go: document GOBIN and 'go install' locations

* In doc/install-source.html, clarify the meaning of $GOBIN and
  describe where executables from the Go distribution are
  installed. Also describe $GOPATH, since it provides a default value
  for $GOBIN and may conflict with $GOROOT.
* Add more detail to 'go help install' as well.

Fixes #31576

Change-Id: Ib8a8c21677c3aa0ebef97a3b587b6f8fe338b80e
Reviewed-on: https://go-review.googlesource.com/c/go/+/182341


Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>