Fix touchpad scrolling on MS-Windows

Fix touchpad scrolling on MS-Windows

* src/w32term.c (w32_construct_mouse_wheel): The number of lines
to scroll should always be positive in wheel-scroll events.
Whether to scroll up or down is encoded in the modifiers, which
produce either wheel-up or wheel-down event.  (Bug#65070)

* doc/lispref/commands.texi (Misc Events): Clarify the
documentation of 'wheel-up' and 'wheel-down' events.