Docs: refine specification of Stream concepts

[akrzemi1]

• Provide the motivation for having ReadSource/WriteSink on top of ReadStream/WriteStream (optimization opportunity).
• Use textual description rather than C++ concepts. C++ cannot cover the requirement "works with any buffer".
• Introduce a formal but not cpp concept that requires things like "works with any buffer" -- syntactic constructs but not checked by a concept.
• Consider either using concept WriteStream<S, Buf> (probably unacceptable) or two bugger archetypes: one for single buffer, one for a range of buffers.

[mvandeberg]

#337 addresses points 1-3. Point 4 still needs to be decided.