Clarify documentation of functions that use the default face
* lisp/simple.el (default-font-height, default-font-width)
(default-line-height): Say in the doc strings that the default
face these functions use is for the currently selected frame.
* doc/lispref/display.texi (Attribute Functions)
(Low-Level Font): Document more clearly the dependence of the
results of 'face-font', 'default-font-width', and
'default-font-height' on the frame's definition of the default
face.