event_waiting_time configuration is not documented properly

[HeshanSudarshana]

Description:
The following configuration is used to provide a delay in the gateway node after an event is received before the data is queried from the control plane.

[apim.event_hub]
event_waiting_time = 2000

This configuration is not mentioned anywhere in the official documentation.

Moreover, the event hub configurations are also missing from the APIM config catalog section of the docs [2].

[1] https://github.com/wso2/carbon-apimgt/blob/master/features/apimgt/org.wso2.carbon.apimgt.core.feature/src/main/resources/conf_templates/templates/repository/conf/api-manager.xml.j2#L1378
[2] https://apim.docs.wso2.com/en/latest/reference/config-catalog/