Improve handling of dedicated flag for side windows (Bug#48493)

Improve handling of dedicated flag for side windows (Bug#48493)

* doc/lispref/windows.texi (Buffers and Windows): Mention the
special handling of side windows and add a reference.
(Buffer Display Action Alists): Say explicitly that
`display-buffer-in-side-window' is dedicating to side by default.
(Dedicated Windows): Add case (4) and explain its meaning, add
a reference.
(Displaying Buffers in Side Windows): Move the paragraph about
`switch-to-(prev|next)-buffer' into a new item to emphasize the
special meaning of dedication for side windows.
* lisp/window.el (set-window-buffer-start-and-point): Restore
side dedication.
(switch-to-prev-buffer, switch-to-next-buffer): Correct return
value that should be nil instead of the same buffer in case of
no change.
(delete-windows-on): Restore side dedication.
(replace-buffer-in-windows): Update the docstring, restore side
dedication.
(quit-restore-window): Rearrange the logic so that strongly
dedicated windows are eventually deleted first.  Restore the
side dedication.  In the final case try to
`switch-to-prev-buffer' before deleting a window (Bug#48367).