doc/go1.16: add release note for disallowing non-ASCII import paths

golang.org/cl/251878 disallowed non-ASCII characters in import paths,
in module mode. They were already disallowed in module paths, so this
change just extended the restriction to the package subdirectory of
the module. Update the release notes to alert users of this change.

Fixes #43052

Change-Id: I1caf9ef978dd3ac599a3f82c5c376ad62e6fc436
Reviewed-on: https://go-review.googlesource.com/c/go/+/282194


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