Improve docstring of 'frame-scale-factor'

* src/frame.c (Fframe_scale_factor): Explain what happens when the
FRAME argument is nil.

src/frame.c

@@ -3747,6 +3747,7 @@ window state change flag is reset.  */)
 DEFUN ("frame-scale-factor", Fframe_scale_factor, Sframe_scale_factor,
        0, 1, 0,
        doc: /* Return FRAMEs scale factor.
+If FRAME is omitted or nil, the selected frame is used.
 The scale factor is the amount by which a logical pixel size must be
 multiplied to find the real number of pixels.  */)
      (Lisp_Object frame)