Add a security domain instructions out of sync with installation information

The installation directions on
https://docs.jboss.org/author/display/MODE50/Installing+ModeShape+into+Wildfly
state that modeshape-users.properties and modeshape-roles.properties are pre 4.0
security-domain config files and no longer needed but page
https://docs.jboss.org/author/display/MODE50/Configuring+ModeShape+in+Wildfly
section, "Step 4a: Add a security domain", describe these files as required for setting
up a security-domain. And the old style security-domain is defined in the CLI cmd.