Rewrite definition for single pointer

The definition for "single pointer" has had issues for a long time, as it mixes the idea of what is a pointer with the action(s) *performed* using a pointer.

See #749 (comment) and the recent #3535 where this is once again causing a non-sequitur

guidelines/terms/21/single-pointer.html

@@ -1,7 +1,5 @@
 <dt><dfn>single pointer</dfn></dt>
 <dd>
-
-   <p>pointer input that operates with one point of contact with the screen, including single taps and clicks, double-taps and clicks, long presses, and path-based gestures</p>
-
+   <p>a single mouse, stylus, or finger on a touch screen, that only targets a single point on the screen (as opposed to multi-pointer/multi-touch scenarios)</p>
 </dd>