Improve documentation of 'expand-abbrev' and wrapper hooks

Improve documentation of 'expand-abbrev' and wrapper hooks

* lisp/simple.el (filter-buffer-substring-functions)
(buffer-substring--filter): Add a link to 'with-wrapper-hook' as
the place to look for documentation of wrapper hooks.
* lisp/minibuffer.el (completion-in-region-functions)
(completion--in-region): Add a link to 'with-wrapper-hook' as the
place to look for documentation of wrapper hooks.
* lisp/abbrev.el (expand-abbrev, abbrev--default-expand): Clarify
the doc strings.  (Bug#24540)

* doc/lispref/text.texi (Buffer Contents): Mention
'with-wrapper-hook's doc string as the place to learn about that
obsolete facility.