Return a pair (BUFFER . POINT) pointing to the definition of FUNCTION.
Finds the source file containing the definition of FUNCTION
in a buffer and the point of the definition. The buffer is
not selected. If the function definition can't be found in
the buffer, returns (BUFFER).
If FUNCTION is a built-in function, this function normally
attempts to find it in the Emacs C sources; however, if LISP-ONLY
is non-nil, signal an error instead.
If the file where FUNCTION is defined is not known, then it is
searched for in `find-function-source-path' if non-nil, otherwise
in `load-path'. (fn FUNCTION &optional LISP-ONLY)