Properly translate touch screen events into mouse drag ones

* doc/lispref/commands.texi (Drag Events): Correct misleading
example form and reword subsequent elaboration.

* lisp/touch-screen.el (touch-screen-handle-point-up)
<mouse-drag>: Set posns representing drag start and terminus to
mouse position lists, in lieu of pairs between windows and posns
employed previously.
1 job for master in 7 minutes and 40 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Normal
failed #74930
test-lisp-inotify

00:07:40

 
Name Stage Failure
failed
test-lisp-inotify Normal
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Cleaning up project directory and file based variables
ERROR: Job failed: exit code 2