Add detailed description for repos2bow and repos2ids subcommands

[zurk]

I already do not remember what the difference and why we have both. I think we need describe it better. The same for classes in https://github.com/src-d/ml/blob/master/sourced/ml/transformers/content2ids.py Cannot get what are they all about?..

[bzz]

🙏 @zurk

Were you thinking about adding it to https://github.com/src-d/ml/tree/master/doc ?