cmd/go: multiple small 'go help' fixes

* Link to privacy policies for proxy.golang.org and sum.golang.org in
  'go help modules'. It's important that both policies are linked from
  the go command's documentation.
* Fix wording and typo in 'go help vcs' following comments in CL 290992,
  which adds reference documentation for GOVCS.
* Fix whitespace on GOVCS in 'go help environment'.

For #41730

Change-Id: I86abceacd4962b748361244026f219157c9285e9
Reviewed-on: https://go-review.googlesource.com/c/go/+/291230


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