doc/go1.16: add multiple release notes for the go command
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 <f3fadaedc813344da5ec13b41e26596889cc5baa@google.com>
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Run-TryBot: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

cloudflare/go,CAFxX/go,CAFxX/go,CAFxX/go,cloudflare/go,golang/go,golang/go,nawawi/go,cloudflare/go,nawawi/go,cloudflare/go,golang/go,CAFxX/go,golang/go,CAFxX/go,CAFxX/go,nawawi/go,golang/go,CAFxX/go,nawawi/go,nawawi/go,cloudflare/go,cloudflare/go,nawawi/go,cloudflare/go,CAFxX/go,golang/go,golang/go,nawawi/go,golang/go,nawawi/go,cloudflare/go