missing attribute on the ResourceBackedMetadataProvider type

[Cantor, Scott]

> That suggests that ResourceBackedMetadataProvider is not used much, if at
> all. In any case, I just removed a large swath of misleading documentation.

We deprecated specific uses of it (File, HTTP) and we deprectated the SVN resource type, so it's all but totally deprecated now in any historical use case, it's mostly left as an extension point since it lets somebody build a fairly customized metadata source with only a Spring Resource subclass.

-- Scott