[webauthn] "might be present on this authenticator" could use a clearer definition

bzbarsky has just created a new issue for 
https://github.com/w3c/webauthn:

== "might be present on this authenticator" could use a clearer 
definition ==
https://w3c.github.io/webauthn/#getAssertion step 7 says:

> execute a platform-specific procedure to determine which, if any, 
credentials listed in allowList might be present on this authenticator

It's not clear to me what this means.  Does the "id" of the 
ScopedCredentialDescriptor need to match something?  Does the type?  
This seems impossible to implement interoperably...

Please view or discuss this issue at 
https://github.com/w3c/webauthn/issues/280 using your GitHub account