Rename ElDoc user option controlling display of truncation notice

Rename ElDoc user option controlling display of truncation notice

The new name makes it consistent with other variables controlling the
display of ElDoc documentation in the echo area.

Per bug#43543.

* etc/NEWS (Eldoc): Rename eldoc-display-truncation-message to
eldoc-echo-area-display-truncation-message.

* lisp/emacs-lisp/eldoc.el
(eldoc-echo-area-display-truncation-message): Rename from
eldoc-display-truncation-message.
(eldoc-display-in-echo-area): Use new variable name.