Updated documentation of point picking event for indexing

 Issue #266
There is no issue here in how it works, we just need a note in pcl_visualizer that picked indices wont correspond to original indices if there are nan's in the cloud... and direct the person to pp_callback in pcd_viewer.cpp for an example of how to find the correct indices.

This patch updates the point picker documentation to reflect that indices won't match up for clouds with NaNs.