Document the maximum length in EcdhPrivateKey.deriveBits for each curve

[jonasfj]

Documentation for EcdhPrivateKey.deriveBits could have bullet point list mentining the maximum length of bits that can be derived from each curve.

Example

[length] specifies the length of the derived secret in bits. The maximum length depends on the [EllipticCurve] used in the keys:
...
 * [EllipticCurve.p521] can provide up to 528 bits.

See:

https://github.com/google/webcrypto.dart/blob/b295d8d0a148e056c285a45c3b38b1ae95ccd34a/lib/src/webcrypto/webcrypto.ecdh.dart#L185-L205

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• [ ] Find maximum length for each curve.
• [ ] Add tests for each curve + maximum length in deriveBits.
• [ ] Document maximum length that can be derived for each curve.