Allow split-window-sensibly to split threshold in further edge case

Allow split-window-sensibly to split threshold in further edge case

As a fallback, and to avoid creating a frame, split-window-sensibly
would previously disregard split-height-threshold if the window to be
split is the frame's root window.

This change generalizes that: it disregards the threshold if the
window to be split is the frame's only _usable_ window (it is either
the only one, as before, or all the other windows are dedicated to
some buffer and thus cannot be touched).

This is required for the fix to bug#28814.

* lisp/window.el (split-height-threshold): Adjust doc to match
split-window-sensibly.
(split-window-sensibly): Also disregard threshold if all other
windows are dedicated.