doc/go1.16: add subheads and adjust formatting in the 'Go command' section
Change-Id: I5f70684d4033d8b11e1cce89268d8222ed596c67
Reviewed-on: https://go-review.googlesource.com/c/go/+/256400
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>