Improve documentation of PointCloud2 row_step, data, and point_step

[tfoote]

This is a follwup to the pre Foxy Message API review

If the point cloud is unordered, is row_step the length of data?

Comment: point_step seems to assumes a fields for a point are adjacent. What if the source data actually has each channel adjacent (all x values, then all y values, then all z values, then all intensities, etc)? Does the data need to be reordered to fit in this message?

What are the possible names of point field? I've seen x, y, z, xyz before, but I don't know of any documentation about those being standard.

https://github.com/ros2/common_interfaces/pull/86/files#diff-4fa5d39b533930879a61949f36d238cbR231

[tfoote]

If the point cloud is unordered, is row_step the length of data?

Yes

point_step seems to assumes a fields for a point are adjacent. What if the source data actually has each channel adjacent (all x values, then all y values, then all z values, then all intensities, etc)? Does the data need to be reordered to fit in this message?

Yes. Won't fix as the commenter came to the right conclusion.

Common field names documented in #112