Clarify purpose of FromDigest trait

[rvolgers]

This is my best understanding from what I was told in another issue.

[tarcieri]

The FromDigest trait is not specifically related to RFC 6979.

The main use case is as a core part of the ECDSA algorithm, however the operation it performs, a hash-to-scalar transformation based on a modular output of a hash function, is the same as the operation described in RFC6979 Section 2.4:

       H(m) is transformed into an integer modulo q using the bits2int
       transform and an extra modular reduction:

          h = bits2int(H(m)) mod q

       As was noted in the description of bits2octets, the extra modular
       reduction is no more than a conditional subtraction.

The ECDSA algorithm itself, separately from RFC 6979, performs a modular reduction of the output of a hash function in order to compute a scalar 𝑒 from H(m), which is subsequently used for both signing and verification.

[rvolgers]

And that's why it should remain in ecdsa instead of being moved to rfc6979, right?

It's good additional context, but I don't see anything actionable for me to change. The RFC6979 explanations of anything are much clearer than the ones in the DSA standard, and the point is to be clear about the expected semantics.

[tarcieri]

Yes, that's why it should be in ecdsa and the rfc6979 crate should consume the reduced outputs.

I think the wording is a bit confusing in that it cites RFC 6979, but unfortunately the primary documentation for ECDSA (e.g. ANSI X9.62) is paywalled.

[tarcieri]

This part can probably be removed