Missing documentation for generic type command

"generic type command" is only mentioned in the management_cli_guide; the actual explanation is in this article: https://developer.jboss.org/docs/DOC-16981;
Hence, the request is:

• use the contents in https://developer.jboss.org/docs/DOC-16981 to add a paragraph to the management_cli_guide
• include explanation for the new "--node-child" option added to "generic type command" to this new paragraph (see RFE https://issues.redhat.com/browse/EAP7-1501 - probably can be taken from https://github.com/wildfly/wildfly-core/blob/73983e1bf4a2c8fa2ff9e2dc238831fb28aed528/cli/src/main/resources/help/command.txt)