DOCS/man/input: fix mouse-pos/hover documentation

The comment about video backends not updating pointer position is only
true if no mouse button is being pressed. If mouse button is pressed on
mpv window and then moves out of window region without releasing the
button, some platforms (win32, Wayland, X11) "capture" the pointer and
still report the coordinates, while some other platforms (SDL) do not.

The value of mouse-pos/hover also differs between platforms in this case.
The value is true on win32 and Wayland, but false on X11.

Fix documentation to address them.