Improve spec of different bitswap versions

[willscott]

currently, it's difficult to find documentation to point to on what the points of incompatibility (if any) are between bitswap 1.1 vs 1.2 (vs 1.0).

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[aschmahmann]

Should be closed by https://github.com/ipfs/specs/pull/269, but if not feel free to open an issue in the specs repo.