Document the "Arbitrary Descriptors" we use in our attribute definitions

Over the years we've added some commonly used "arbitrary descriptors" for management attributes and params. Cover these in the docs at [1]. Finding uses of SimpleAttributeDefinitionBuilder.addArbitraryDescriptor is a good way to identify these. Perhaps not all uses of that should be documented, but the ones that represent a standard way we want to handle certain things should be.

[1] https://github.com/wildfly/wildfly/blame/master/docs/src/main/asciidoc/_admin-guide/management-api/Description_of_the_Management_Model.adoc#L305