bug#18296: 24.4.50; doc string of `whitespace-mode'

[Drew Adams]

> > 1. Please mention which faces are used for which kinds of highlighting.
> 
> Well, there's a lot of faces, and I think it's unusual for modes like
> this to enumerate them.

There are N faces, nicely documented in `whitespace.el'.
Why not put this info in the doc?

`whitespace-space'            highlight SPACE
`whitespace-hspace'           highlight HARD SPACE
`whitespace-tab'              highlight TAB
`whitespace-newline'          highlight NEWLINE char mapping
`whitespace-trailing'           highlight trailing blanks
`whitespace-line'             highlight "long" lines
`whitespace-space-before-tab' highlight SPACEs before TAB
`whitespace-indentation'      highlight 8+ SPACEs at bol
`whitespace-big-indent'       highlight big indentation
`whitespace-empty'            highlight empty lines at bob/eob
`whitespace-space-after-tab'  highlight 8+ SPACEs after TAB

(Quickly abbreviated from the commentary - not suggesting
that those are the best descriptions.  E.g., what is a newline
char mapping? what is "big" indentation?)

> But perhaps a link to the customisation group would be nice?

That would be OK too.  But the mode help is where I think
it belongs.

> Is there a syntax for that?

Dunno - not that I can think of.