Specify the compression window sizes for zstd-d and br-d

[pmeenan]

This change makes it explicit that the zstd-d content encoding is Zstandard with up to a 8 MB compression window (same as zstd) and br-d is Brotli with up to a 16 MB compression window (same as br).

See the discussion in #2754 for why larger ZStandard values were not selected.

This carries some nuance for what encoding to use when compressing for maximum benefit as the resource size crosses beyond 8 MB but that is more of an editorial discussion about the individual compression algorithms and probably better laid out in blog posts or explainers than in the spec.

Fix #2754

[pmeenan]

@nidhijaju could you PTAL and make sure the language matches what you're working on for the zstd update?

[pmeenan]

Thanks. I switched the content-encodings definitions and IANA registrations to a list with references back to the algorithm RFCs. For the IANA registrations I removed the window size from the text and linked it to this document (where the window sizes are defined) instead of to the compression algorithms directly.

Let me know if a more verbose description of the need to specify the window sizes would be useful or if just being explicit about what they are is enough.

[nidhijaju]

lgtm other than the comment