bug#3699: Emacs manual revision for whitespace.el

[Chong Yidong]

Hi Vinicius,

Could you suggest a change to the Emacs manual that incorporates
information about the whitespace package?  Thanks.

> (info "(emacs) Useless Whitespace") should mention whitespace.el items.
>
> show-trailing-whitespace is a variable defined in `frame.el'... maybe
> its docstring should mention whitespace.el existence... or maybe it
> should be merged/moved into that file?
>
> And whitespace.el should say what the difference between "HARD" and
> non-HARD whitespace is.