improve http2-encoded-data for -04

* better abstract
* expand "HTTP/2" in abstract and intro
* clarify that ACCEPT_ENCODED_DATA always has stream_id=0
* add ENCODING_IDENTITY, with default rank=1 that is always acceptable
* rewrite the bits about decoding/re-encoding forwarded data, and segmentation
* take more "boilerplate" text from DATA, about stream-id, padding, etc.
* add a couple of new section anchor/ids
add more abstract to http2-segments
rebuild XML from markdown