Replace eldoc-documentation-function with a hook

Replace eldoc-documentation-function with a hook

* lisp/emacs-lisp/eldoc.el (eldoc-documentation-functions): New hook.
(eldoc-documentation-function): Make into obsolete alias.
(eldoc-echo-area-use-multiline-p, eldoc-highlight-function-argument):
(eldoc-argument-case, global-eldoc-mode): Update docstrings.
(eldoc-print-current-symbol-info): Use run-hook-with-args-until-success
on eldoc-documentation-functions.
(eldoc-supported-p): New function.
(eldoc-mode, global-eldoc-mode, eldoc-schedule-timer): Use eldoc-supported-p.
* etc/NEWS: Mention eldoc-documentation-functions.
* doc/lispref/modes.texi: Update reference.