Merge branch 'ma/sha-256-docs'

Further update of docs to adjust to the recent SHA-256 work.

* ma/sha-256-docs:
  shallow.txt: document SHA-256 shallow format
  protocol-capabilities.txt: clarify "allow-x-sha1-in-want" re SHA-256
  index-format.txt: document SHA-256 index format
  http-protocol.txt: document SHA-256 "want"/"have" format