Reword note about primary/non-primary pointers

[patrickhlauke]

• some browsers seem to "promote" pointers to be primary when the original primary leaves the screen. as we leave this up to the user agents currently, softened the language to "may"
• generalised the sentence about primary pointers outside of the user agent, as it was a bit too specific about this being for touch (which yes, in practice is the only realistic scenario, but because we're allowing - at least theoretically - for any kind of pointerType to have more than one simultaneous pointer, this makes more sense generalised as well).

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