Document changes to windows code.

Document changes to windows code.

* frames.texi (Frames and Windows): Move section and rename to
Windows and Frames in windows.texi.
* windows.texi (Windows): Restructure.
(Basic Windows): Rewrite.  Explain live and internal windows and
normalization functions.
(Windows and Frames): Move section here from frames.texi.
Describe subwindows, window combinations, window tree, and
corresponding functions including window-list here.
(Window Sizes): Rename section from Size of Window and move it
up in chapter.  Describe total and body sizes and the
corresponding functions.  Explain new semantics of
window-min-height/-width.
(Resizing Windows): Move section up in chapter.  Describe new
resize functions.
(Splitting Windows): Describe new behavior of split-window,
split-window-above-each-other and split-window-side-by-side.
Provide examples.  Describe window-nest and window-splits
options.
(Deleting Windows): Minor rewrite.
(Selecting Windows): Minor rewrite.  Describe
frame-selected-window and set-frame-selected-window here.
(Cyclic Window Ordering): Minor rewrite.  Describe
window-list-1.
(Buffers and Windows): Rewrite.  Explain a window's previous and
next buffers and the corresponding functions.
* elisp.texi (Top): Update node listings for frames and windows
sections.