Add reference to #sctn-discover-from-external-source from discussion …

…of get() internals

index.bs

@@ -2321,6 +2321,7 @@ the user select a [=public key credential source=].
 Since this specification requires an [=authorization gesture=] to create any [=assertions=],
 {{PublicKeyCredential}} inherits the default behavior of
 {{Credential/[[CollectFromCredentialStore]](origin, options, sameOriginWithAncestors)}}, of returning an empty set.
+{{PublicKeyCredential}}'s implementation of {{PublicKeyCredential/[DISCOVER-METHOD]}} is specified in the next section.
 
 In general, the user agent SHOULD show some UI to the user to guide them in selecting and authorizing an authenticator with which
 to complete the operation. By setting <code>|options|.{{CredentialRequestOptions/mediation}}</code> to {{CredentialMediationRequirement/conditional}}, [=[RPS]=] can indicate that a prominent modal UI should <i>not</i> be shown <i>unless</i> credentials are discovered.