Strictly this should be .B as there's no argument so the I in .BI isn't needed...

This option might be better placed after -t so the ordering-based options are closer together. It should also be mentioned in the "The ordering of the records in the input files must match..." paragraph.

I'll push a commit here to make the changes above. There are some other improvements that could be made (e.g. moving -o) but they may be better done separately.