Add Admin Guide section on using expressions in the configuration

Reading the discussion on WFLY-9452 I realized that our docs don't explicitly cover how management model expressions work! Stuff like expression resolution sources, how to determine if an attribute supports and expression, the fact that updating the resolution source does not affect previously resolved values, etc.

It would be good to have something about the ConfigMap scenario covered in the WFLY-9452 discussion as well.

A section under http://docs.wildfly.org/15/Admin_Guide.html#Management_tasks seems like a reasonable spot.