doc: architecture.txt: start updating architecture.txt

doc: architecture.txt: start updating architecture.txt
I chose to remove mention of non-convergent encoding, not because I dislike non-convergent encoding, but because that option isn't currently expressed in the API and in order to shorten architecture.txt.  I renamed "URI" to "Capability".  I did some editing, including updating a few places that treated all capabilities as CHK-capabilities and that mentioned that distributed SSKs were not yet implemented.