update -manageability for BKK spin bit consensus

[britram]

Review and update text in -manageability to reflect BKK spin bit consensus:

• single spin bit is in the protocol spec now
• endpoints may opt out (discuss why; this is for any reason, not just proxies as suggested in For privacy concerns, the spin bit may be disabled in specific use cases. #55),
• endpoints should probabilistically out out to provide cover for endpoints opting out

[loganaden]

I am working on this.

[loganaden]

@britram what about having a privacy consideration section ?

[britram]

I'm not sure this fits in the scope of the document: can you say more about what you propose to be in this section? The -manageability document is meant to be an independent user's guide to the wire image, so information on the spin bit here will be more about how to measure it, not additional guidelines on how to generate it and when not to (though such text might belong in -applicability)

Please coordinate with @ihlar, who will also be proposing text on spin measurement to bring over from draft-ietf-quic-spin-exp.

[mirjak]

see PR #66

[britram]

closed by merge of #66 and #67