explanation of site_annotations output

[EricRLucas]

Please could the the docs include information on how to interpret the output of site_annotations()? I can't work out what the different levels of codon_nonsyn mean (effect?). I also can't quite work out what degeneracy shows (all 4-fold degenerate positions have a 0 score for nonsyn, which makes sense, but none of the other degeneracies have a 0; I would have thought that a 3-fold degenerate position would have a lot of 0 values).

[leehart]

Internal ref: https://github.com/malariagen/vector-ops/issues/1278