add Note wrt 'alg', thx jyasskin

index.bs

@@ -975,6 +975,9 @@ optionally evidence of [=user consent=] to a specific transaction.
 
     The <dfn>alg</dfn> member specifies the cryptographic signature algorithm with which the newly generated credential
     will be used, and thus also the type of asymmetric key pair to be generated, e.g., RSA or Elliptic Curve.
+
+    Note: we use "alg" as the latter member name, rather than spelling-out "algorithm", because it will be serialized into
+        a message to the authenticator, which may be sent over a low-bandwidth link.
 </div>
 
 ## Options for Credential Creation (dictionary <dfn dictionary>MakeCredentialOptions</dfn>) ## {#dictionary-makecredentialoptions}