To install this package, run in Emacs:
M-x package-install RET eldoc RET
This program was inspired by the behavior of the "mouse documentation window" on many Lisp Machine systems; as you type a function's symbol name as part of a sexp, it will print the argument list for that function. Behavior is not identical; for example, you need not actually type the function name, you need only move point around in a sexp that calls it. Also, if point is over a documented variable, it will print the one-line documentation for that variable instead, to remind you of that variable's meaning. This mode is now enabled by default in all major modes that provide support for it, such as `emacs-lisp-mode'. This is controlled by `global-eldoc-mode'. Major modes for other languages may use ElDoc by adding an appropriate function to the buffer-local value of `eldoc-documentation-functions'.