Fix affected signal and docs

Fix affected signal and docs

Signal wasn't being called in its primary usecase, which was to interact
without altering the particles. Docs have been updated to clarify the
usecase as well.

This also updates the position properties of custom affector to more
accurately specify if they caused changes.

Change-Id: Ia411d9e81002443a56ed8bab9232f1aad49a4c5e
Reviewed-by: Yunqiao Yin <charles.yin@nokia.com>