doc/go1.16: add multiple release notes for the go command

Added notes for:

* go test -c and -i flags used with unknown flags
* GO111MODULE=on by default
* GOVCS
* Dropped requirements on excluded versions

Removed TODOs for documentation on the retract directive and
'go install pkg@version'. These pages will be written after the beta.

Change-Id: Ic9877a62f908be177a6035a039b72e969e7b7f22
Reviewed-on: https://go-review.googlesource.com/c/go/+/274438


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