Document consuming multipart entities

Not only for the server-side (i.e. for consuming file-uploads) but also for the client-side where servers may return `multipart/byteranges` or other multipart datastructures.

Information it should contain:
* how to use `Unmarshal(response / entity).to[Multipart.General]`
* other subclasses of `Multipart`
* explain stream-of-streams nature of `Multipart.parts` and `Multipart.BodyPart.entity.dataBytes`
* note about head-of-line blocking: the data of a part needs to be read fully before a new part will be dispatched. Just filtering parts can deadlock a stream.

Locate the documentation entry point for multipart entities and start by reviewing Unmarshal(response / entity).to[Multipart.General] and the other Multipart subclasses. Explain the stream-of-streams behavior of Multipart.parts and Multipart.BodyPart.entity.dataBytes, including the head-of-line blocking warning. Done means the documentation covers both server-side uploads and client-side multipart responses such as multipart/byteranges.

Written by the indexing model from the issue text.