tablet: Clarify what happens when set_cursor is not called

clients can use set_cursor to set a cursor surface for tablet cursors (or alternatively use cursor-shape to set a shape cursor). However compositors differ in their behavior when the client never sets a cursor:

• Weston shows a separate tablet cursor that looks like the mouse cursor
• Sway seems to have a unified mouse/tablet cursor, so the tablet cursor gets the appearance of the mouse cursor
• KWin has a separate tablet cursor, with a custom fallback appearance
• Gnome/Mutter does not have a visible tablet cursor (but tablet interaction still works)

While arguably the decision of whether tablet and mouse cursor should be separate or not should be compositor policy, IMO it would be good to clarify in the protocol what behavior wrt the cursor appearance can be expected when not explicitly specifying a cursor.

The closest thing in the protocol I can find is in set_cursor: "If surface is NULL, the cursor image is hidden". But that would only apply to when calling set_cursor with NULL, not when never calling set_cursor.

See https://bugreports.qt.io/browse/QTBUG-105843 for where this came up