Window-related updates to NEWS and Emacs manual.

Window-related updates to NEWS and Emacs manual.

* etc/NEWS: Reorganize, placing user options in Changes section.

* doc/emacs/buffers.texi (Select Buffer): Clarify description of
buffer-switching commands.  Add xref to Window Display node.
Don't repeat confirm-nonexistent-file-or-buffer description from
Visiting node.  Remove even-window-heights.

* doc/emacs/frames.texi (Special Buffer Frames): Add xref to Window Choice.

* doc/emacs/windows.texi (Pop Up Window): Defer discussion of window
splitting to the Window Choice node.  Add index entries.
(Force Same Window): Node deleted.
(Displaying Buffers, Window Choice): New nodes.