Function: eldoc-mode

Toggle echo area display of Lisp objects at point (ElDoc mode).
With a prefix argument ARG, enable ElDoc mode if ARG is positive,
and disable it otherwise. If called from Lisp, enable ElDoc mode
if ARG is omitted or nil.

ElDoc mode is a buffer-local minor mode. When enabled, the echo
area displays information about a function or variable in the
text where point is. If point is on a documented variable, it
displays the first line of that variable's doc string. Otherwise
it displays the argument list of the function called in the
expression point is on. (fn &optional ARG)