New Eldoc function `eldoc-show-help-at-pt'

New Eldoc function `eldoc-show-help-at-pt'

Show `help-at-pt' string via Eldoc as an alternative to the
`help-at-pt-display-when-idle' timer.  The help-at-pt timer
competes with Eldoc for the echo area, such that the two
mechanisms do not work well together.  Therefore when using
Eldoc, the setting `eldoc-help-at-pt' may be preferable.
* lisp/emacs-lisp/eldoc.el (eldoc-help-at-pt): New customization option.
(eldoc-show-help-at-pt): New Eldoc function.
(eldoc-documentation-functions): Register the new function.
* lisp/help-at-pt.el (help-at-pt-display-when-idle): Mention
`eldoc-help-at-pt' in the docstring.
* doc/emacs/help.texi: Document `eldoc-help-at-pt'.
* etc/NEWS: Announce the change.  (Bug#77169)

(cherry picked from commit ab71699e5f2502aff6c65dd195611cfbbe2f2255)