go/types: remove a TODO, enable a test

1) Scopes do have a comment field for documentation (debugging output).
   No need to do anything extra.

2) The testcase in expr3.src has ok error messages. Enabled.

Change-Id: Ic1a03bfec0a6a70d876aa6cfb936973abe58fe6c
Reviewed-on: https://go-review.googlesource.com/c/139902


Reviewed-by: Alan Donovan <adonovan@google.com>