Document OneHotArray

[cossio]

It seems that the new OneHotArray (#1448) is not documented. There is nothing mentioned in https://fluxml.ai/Flux.jl/dev/data/onehot/ and it's not clear how one should use OneHotArray. Can we add some documentation? @darsnack

[darsnack]

Yup the docs for the old onehotbatch etc. should be there, but I'll add more docs around the new type. (instead of just docstrings)