Use RFC2119 for defined extensions (#1134)

Author: plehegar

Diff for: index.bs

@@ -4367,7 +4367,7 @@ There MUST NOT be any values returned for ignored extensions.
 
 This section defines the initial set of extensions to be registered in the
 IANA "WebAuthn Extension Identifier" registry established by [[!WebAuthn-Registries]].
-These can be implemented by user agents targeting broad interoperability.
+These MAY be implemented by user agents targeting broad interoperability.
 
 
 ## FIDO <dfn>AppID</dfn> Extension (appid) ## {#sctn-appid-extension}