Clarify NestedConfigurationProperty doc with regards to Collections/Maps

[snicoll]

See #9894

@NestedConfigurationProperty (singular) is about asking the annotation processor to crawl a POJO as a nested type, creating a group with the same name as the property exposing the pojo. We should probably say that at the annotation level and in the doc. In particular, mentioning this has no effect for a list of objects or map of objects would be a good idea.