MappedAttributeInMetadata Filter

[Rod Widdowson]

Now I come to document it, this seems to pretty much overlap with the
AttributeInMetadata Filter, only without the latter's configurability.  Do
we want to document this or deprecate it?

If the latter, then we could ask some questions about AttributeMapping.
However I suspect that this is a feature which we may want to make more use
of in the future.

Rod