Describe window size preserving options.

* windows.texi (Resizing Windows): Describe new argument of
`fit-window-to-buffer'.  Move description of `window-size-fixed'
to new section below.
(Preserving Window Sizes): New section describing
`window-size-fixed' and `window-preserve-size'.
(Display Action Functions): Describe `preserve-size' alist
entry.
(Window Parameters): Describe `preserved-size' parameter.
In NEWS mark `preserve-window-size' as +++.