Add "modality" to single pointer definition with examples of single pointer interactions (#4070)

Follow-up to
#3536 (comment) to
potentially address concerns by @WilcoFiers @dbjorge

Tries to more clearly differentiate "input" as in the *modality* from
the gestures/interactions that are the *result* of using a particular
input *modality*


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title="Last updated on Sep 20, 2024, 8:10 PM UTC (a79bb49)">Preview
| https://pr-preview.s3.amazonaws.com/w3c/wcag/4070/4213bf6...a79bb49.html"
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Co-authored-by: Alastair Campbell 
Co-authored-by: Mike Gower 
(cherry picked from commit b9cf8f8)

Author: 3 people

guidelines/terms/21/single-pointer.html

@@ -1,6 +1,6 @@
 <dt><dfn id="dfn-single-pointer">single pointerdfn>dt>
 <dd>
-   <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>
+   <p>an input modality 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">Single pointer interactions include clicks, double clicks, taps, dragging motions, and single-finger swipe gestures. In contrast, 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>