Update definition for single pointer (#3536)

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

I originally tried to fix this, but there was no appetite for it once
2.1 was released. However, with 2.2 and the new 2.5.7 Dragging Movement
SC, the broken definition is causing actual misunderstandings/illogical
non-sequiturs.

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

Closes #3535

(this is effectively a follow-up to #809
which had disambiguated things, but the definition had since been
changed further/again to reintroduce the ambiguous wording we have at
this point which confuses input with action)

This would be applied to WCAG 2.1 and 2.2, unless there is a decision to
only apply it to 2.2.

EDIT: Also closes #394


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---------

Co-authored-by: Alastair Campbell <ac@alastc.com>

guidelines/terms/21/single-pointer.html

@@ -1,7 +1,6 @@
 <dt><dfn id="dfn-single-pointer">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>an input that only targets a single point on the page/screen at a time – such as a mouse, single finger on a touch screen, or stylus.</p>
+   <p class="note">In contrast to single pointer inputs, multipoint interactions involve the use of two or more pointers at the same time – such as two finger interactions on a touchscreen, or the simultaneous use of a mouse and stylus.</p>
 </dd>