Document the fields in pcl::PCLHeader.

Especially the interpretation of the timestamp field is important.
Merge pull request #684 from j-oberlaender/doc_improvements

Document the fields in pcl::PCLHeader.
Fixed a small bug

Forgot pcl:: from the function declaration
Merge branch 'master' of https://github.com/PointCloudLibrary/pcl into bilateral_upsampling_fast