bug#17569: 24.4.50; doc of "wrapper hooks"

[Drew Adams]

> > OK, thanks for the history.  But if those variables whose doc
> > refers to "wrapper hook" still have values that are a list of
> > functions that are, in fact, wrapper hooks, then the concept
> > still needs to be explained, one way or another.
> 
> No, such vars should be obsolete as well (and hence removed
> from the doc).

So not only is something like `buffer-substring-filters' to be
considered obsolete (the case since Emacs 24), but also a var
like `filter-buffer-substring', whose doc refers to the fact
(since Emacs 24) that it uses "obsolete variable
`buffer-substring-filters'."

And what about a function, such as `filter-buffer-substring',
whose implementation and doc string makes use of such a var?
What is being recommended now for someone who wants the
functionality of `filter-buffer-substring' - just roll your own?