Clarify the uses of ByteStream vs Builder

[andrewthad]

These two types are sort of similar, but they are different. ByteStream is effectful. Consuming it often causes file handles to move. Its main purpose is reading bytes from disk or from the network into memory, and these bytes will typically be parsed as they arrive. Builder is, in a sense, the opposite. It's for serializing objects that live in the heap. The builder type itself is not effectful, but it can be interpreted effectfully. This can be used, for example, to stream bytes to disk as they are produced. I need to add this information to the docs.