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

[Drew Adams]

> I documented this for an earlier release.
> Then it was made obsolete, and the documentation deleted, without
> cleaning up references to that documentation.

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.