SDW message should include identifier of the warning for suppressing it

Without access to Daffodil internals, you don't have enough information to suppress a warning in a config file, because you don't have the necessary identifier. 

When we print an SDW, this identifier should be printed. 

Suggest on a separate line explaining how to suppress it. 

"To suppress, add 'warningIdentifier' to the daf:suppressSchemaDefinitionWarnings element of a Daffodil defineConfig used when processing."

Or something like that.