Clarify language around public key expression constraints

index.html

@@ -1572,8 +1572,12 @@ <h3>Key types and formats</h3>
         </p>
 
         <p>
-Public key expression MUST NOT use any other key format than those listed in
-the <a href="#public-key-support">Public Key Support table</a>.
+When using any of the public key types described here, public key expression
+MUST NOT use any other key format than those listed in the
+<a href="#public-key-support">Public Key Support table</a>. For public key
+types that are not listed here, the type value and corresponding format
+property SHOULD be registered in [[DID-SPEC-REGISTRIES]], as with any other
+verification method.
         </p>
 
         <p class="issue">