Fixes #16258.

Docs for Encode and EncodeValue do not mention that
nil pointers are not permitted hence we panic,
because Gobs encode values yet nil pointers have no value
to encode. It moves a comment that was internal to EncodeValue
to the top level to make it clearer to users what to expect
when they pass in nil pointers.
Supplements test TestTopLevelNilPointer.

Change-Id: Ie54f609fde4b791605960e088456047eb9aa8738
Reviewed-on: https://go-review.googlesource.com/24740Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Andrew Gerrand <adg@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>