Lacking important information in item 42.1. Protocol Interoperability

In order to configure protocol interoperability (cache compatibility mode), the documentation in https://access.redhat.com/documentation/en-us/red_hat_jboss_data_grid/7.1/html-single/administration_and_configuration_guide/#protocol_interoperability is vague, and leads to the use of both:

<compatibility/> (when not using custom marshaller) or
<compatibility marshaller="..."/> (when using custom marshaller)

When enabling this feature for use within OpenShift, QE has reported it did not work as expected. Investigating, we found out that the use of the "enable" attribute is also mandatory, as confirmed by gustavofernandes in CLOUD-1863.

So, the documentation could show as examples:
<compatibility enabled="true"/> or
<compatibility enabled="true" marshaller="com.acme.MyMarshaller"/>

Also of note: as of today, there is no support for configuring external custom marshallers, despite working if it's deployed as a custom module. So, it's necessary to add both the feature and the proper documentation about how to do so. Related: JDG-1026.