Document RRB row mask function

[gardners]

Describe where we can find the problematic topic
A clear and concise description of what the problem is. Ex. Chapter/Section/Subsection.
Refrain from using page numbers, better refer to the TeX File and line number.

Describe the solution you'd like
A clear and concise description of what you want to happen.
Perhaps you could make Pull Request with the solution instead of reporting an issue?

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

[dansanderson]

@gardners Do you have any notes or references that can replace the placeholder bug description?