Essential docs for more comfort code navigation

[donRumata03]

It's a common practice to put short (only essential) docs in __init__.py files with general information what is the module responsible for.
That seems to be extremely useful for new contributors since it provides abstractions from authors' heads directly to docs while being pretty easy to create (only short docs are required).

Also, essential details of implementation of undocumented classes could be useful for the same reasons (full docs with listing all the parameters indeed seem redundant).