Re: [webauthn] clarify content of algorithm member of copedCredentialParameters

The words "desired" and "best effort" led me to believe that 
`cryptoParameters` was just a guide, and that if no match in 
`cryptoParameters` was found then any alternative credential would be 
acceptable. I would suggest clarifying that if no match is found in 
`cryptoParameters` an error is returned.

Returning to your question about how much detail an RP App should 
provide in specifying a credential, isn't this already addressed by 
the 
[definition](https://www.w3.org/TR/WebCryptoAPI/#algorithm-dictionary)
 of `AlgorithmIdentifier` which may be either a string or an object? 
The object can contain all the details you want and if it's a string 
like "RSASSA-PKCS1-v1_5", then the [normalizing 
algorithm](https://www.w3.org/TR/WebCryptoAPI/#algorithm-normalization)
 will fill in the details?

PS - Hopefully it's obvious, but we may want to specify that the 
`algorithm` must be one that supports the `sign` and `verify` methods,
 as described in [WebCrypto Section 
19](https://www.w3.org/TR/WebCryptoAPI/#algorithm-overview).

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