fix: add comment about EC algorithm support in GdchCredentials

Author: diegomarquezp

oauth2_http/java/com/google/auth/oauth2/GdchCredentials.java

@@ -233,6 +233,7 @@ static GdchCredentials fromJson(Map<String, Object> json, HttpTransportFactory t
    */
   static GdchCredentials fromPkcs8(String privateKeyPkcs8, GdchCredentials.Builder builder)
       throws IOException {
+    // GDCH key generation natively only supports the EC algorithm.
     PrivateKey privateKey =
         OAuth2Utils.privateKeyFromPkcs8(privateKeyPkcs8, OAuth2Utils.Pkcs8Algorithm.EC);
     builder.setPrivateKey(privateKey);