Add Framer to Figure 3

[mirjak]

Reading something about framers in section 4.1.4 came actually a bit as a surprise to me as we don't talk a lot about framer earlier in the document (yes there is one forward reference in section 2.2 but that's it). Now that we have message in Figure 4, we should maybe also add framers...?

[mirjak]

Also maybe we should say a bit more what framers are than we currently do?

The current text is:
" * Framer: A Framer is a data translation layer that can be added to
a Connection to define how application-layer Messages are
transmitted over a transport protocol. This is particularly
relevant for protocols that otherwise present unstructured
streams, such as TCP."

We could probably add to this:
"A framer can define a bit pattern or syntax such as a length field or prelimiter that is pre- or suspended by the transport service to the message's bytes and is know by the transport service or application on the other endpoint to parse the received bytes into a message."

[tfpauly]

Framer goes at bottom of API above implementation