spec: complete list of special comma-ok forms

spec: complete list of special comma-ok forms

The enumerations didn't include the syntactic form where the lhs is
full variable declaration with type specification, as in:

var x, ok T = ...

Fixes #15782.

Change-Id: I0f7bafc37dc9dcf62cdb0894a0d157074ccd4b3e
Reviewed-on: https://go-review.googlesource.com/27670
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Rob Pike <r@golang.org>