Rewrite "Size and Position" section in frames.texi.

Rewrite "Size and Position" section in frames.texi.

* frames.texi (Size Parameters): Replace "frame contents" by
"frame's text area".  Add reference to Size and Position
section.
(Size and Position): Major rewrite.  Add explanations for
frame's default font, text and display areas.  Add descriptions
for `set-frame-font', `frame-text-height', `frame-text-width'
and `frame-inhibit-implied-resize'.