Add docs for valid uses of Pointer.
Then document change made for #13372 in CL 18584.

Fixes #8994.

Change-Id: Ifba71e5aeafd11f684aed0b7ddacf3c8ec07c580
Reviewed-on: https://go-review.googlesource.com/18640Reviewed-by: Alan Donovan <adonovan@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Rick Hudson <rlh@golang.org>