ElDoc: make eldoc-display-in-echo-are useful from M-x eldoc

ElDoc: make eldoc-display-in-echo-are useful from M-x eldoc

M-x eldoc is ElDoc's interactive entry point for on-demand
documentation for users that don't want the behind-the-scenes idle
timer behaviour.

However, eldoc-display-in-echo-area, a member of
eldoc-display-functions, refused to do anything because it thought it
didn't have permission to use the echo area, which isn't true
in interactive use cases.  Fix that.

See also: https://github.com/joaotavora/eglot/discussions/1328

* lisp/emacs-lisp/eldoc.el (eldoc-display-in-echo-area): Use
INTERACTIVE argument.  Rework comments.
(Version): Bump to 1.15.0