Describe bucket_id meaning in documentation

[olegrok]

Sometimes our customers don't understand that bucket_id it's an only abstraction between physical node and single storage contains not only single bucket_id. In other words "bucket_id" as option it's not a filter it's an option for routing.

So, it would be great to add some note about it. E.g. crud.get with different bucket_id could return correct result because this two bucket_ids (of returned object and user defined) point the same storage.

[Totktonada]

There is also a request to change the bucket_id option meaning: #190.

However I vote for describing current meaning of the option first.