Fix documentation per bug #9949.

Fix documentation per bug #9949.

 src/window.c (Fwindow_body_size): Mention in the doc string that the
 return value is in frame's canonical units.
 lisp/window.el (window-body-height, window-body-width): Mention in
 the doc string that the return values are in frame's canonical
 units.
 doc/lispref/windows.texi (Window Sizes): Mention in the doc string that the
 return values of `window-body-height' and `window-body-width' are
 in frame's canonical units.