ace-window.el (aw-make-frame): Fix height using cdr of aw-frame-size

[skalawag]

The height parameter was incorrectly using (car aw-frame-size), the
width component, instead of (cdr aw-frame-size). This caused any
explicit height value to be silently ignored, with the width value
used in its place.

Also correct the aw-frame-size docstring: the units are characters
and text lines (as documented by make-frame), not pixels.

Reproduction:

#+begin_src emacs-lisp
;; Set a frame size where width and height are visibly different.
(setq aw-frame-size '(120 . 40))
#+end_src

Then, with at least two windows visible, invoke ace-window and press
z (or whichever character aw-make-frame-char is bound to). A new
frame will appear. Inspect its dimensions:

#+begin_src emacs-lisp
(cons (frame-width) (frame-height)) ; run in the new frame
#+end_src

Expected: (120 . 40) --- width 120 columns, height 40 lines.

Actual: (120 . 120) --- both dimensions are the width value because
(car aw-frame-size) (120) is used for height instead of
(cdr aw-frame-size) (40).