Record Validation Filter Documentation gaps

[k-wall]

Currently the Record Validation Filter documentation is quite light. It merely illustrates how to configure the filter to apply a JSONSchema validation against a records produced to a topic.

Gaps:

1. It doesn't discuss how the filter reacts when a validation fails. Nor does it mention how the option to forwardPartialRequests can be used to reject just the failed records or how the rules apply when transactions are in-use.
2. It doesn't talk about the different validation types (SyntacticallyCorrectJson and SchemaValidation) and how the validations work together.
3. It doesn't talk about allowNulls / allowEmpty.
4. It doesn't document that SchemaValidation currently only works with JSONSchema (that is Avro etc is not supported). (Support for schema types other than JSON #1431)
5. It doesn't document the defaultRule.
6. It doesn't mention that Apicurio is the supported registry.