Improve feature provider option descriptions

At this point the [descriptions of the plugin options](https://github.com/lgeiger/ide-python/blob/master/package.json#L67-L350) visible in the Atom settings are rather basic and often less accurate than what can be found in the documentation of the specific feature provider.

We should improve those descriptions and/or link to the relevant docs if applicable.

Any help on this is highly appreciated. This issue doesn't need to be fixed in one go, we can iteratively improve the plugin documentation.