Document device features

[rytilahti]

Update documentation (design.rst) concerning the newly added device features.

[sdb9696]

There's a few ways to do this but I think it would be best if the documentation is embedded in the code somehow. Do we want per feature documentation or just an overview of how a feature works? If it's per feature we could either add to the modules that create the features or inline it with each feature in the list of feature ids here: #926.

[rytilahti]

Just a quick, general overview of how modules & features work in the separate doc is enough, the details (where needed) would be inside docstrings.

Basically just discussing how the interfaces are designed and expected to be used.

[sdb9696]

Done in #968