General suggestions for BIP cleanup

[apoelstra]

This looks great to me! I think @roconnor-blockstream may take issue with the "such as" language in the first paragraph, since I think this scheme is supposed to be used specifically with BIP-32 seeds.

[robot-dreams]

@apoelstra sounds good, updated to make it clear it's specifically for BIP-32

Added a bit more copy editing. One thing to note, previous we had this terminology:

• An MS32 string with share index != s is called an MS32 share
• An MS32 string with share index = s is called an MS32 secret
• The component of the "data part" that actually contains the Bech32 encoded data is called a secret share

I think this could cause some confusion—in the PR I renamed the last thing to payload instead, but LMK if you prefer a different name.

[apoelstra]

Thanks, this all looks great to me!