Update description of security-realm in management interfaces

Description of security-realm attribute in HTTP (and native) management interfaces says:
"The security realm to use for the HTTP management interface." or "The security realm to use for the native management interface."

However Elytron also uses security-realms. Description of mentioned about attribute should include information that it uses legacy security realm (not Elytron). Without word "legacy" it can be confusing since two different types of security-realm currently occur in configuration.