Editorial cleanup on terms

I've done an editorial cleanup pass on the WCAG 2.1 term definitions, 
shown at:

https://github.com/w3c/wcag21/commit/6d3cd89f484ed9842f80284d4e8c86965ee3b6ea

I believe these to be purely editorial changes; I filed a couple issues 
for things I saw that weren't  editorial.

The edit to single-pointer activation might raise the most eyebrows, 
because I took the first sentence "One point of contact with the screen 
(vs. multi-touch)." and made it "one point of contact with the screen" 
and then started the next paragraph with "This is in contrast to 
multi-touch.". This is to fit the pattern where the first line of term 
should be a phrase that in principle could drop in place of the term 
within the SC text, and was a little less straightforward than othe terms.

Michael