NF+DOC: minor convenience function: mouse.isPressedIn()

The motivation is to enable intuitive expressions like
   if mouse.isPressedIn(polygon):
   while not mouse.isPressedIn(shape):
This allows one to avoid
   if any(mouse.getPressed()) and shape.contains(mouse):
   while not any(mouse.getPressed()) or not shape.contains(mouse):
- Could work with ImageStim, but ImageStim.contains() needs work first.
- I chose "isPressedIn" to match "getPressed"

Also fixed two typos in doc-strings, and adjusted event.rst.