getFromBrowseableDirectories unclear about path

Noticed in this forum thread: https://discuss.lightbend.com/t/getfrombrowseabledirectories/6029

[The docs sample](https://doc.akka.io/docs/akka-http/current/routing-dsl/directives/file-and-resource-directives/getFromBrowseableDirectory.html) covers the browsing part but uses `path("bbb")` without mentioning you will need to use `pathPrefix` for actually browsing contents.

Start with the linked documentation sample for getFromBrowseableDirectory and compare its use of path("bbb") with the reported browsing behavior. Update the documentation to explain when pathPrefix is needed for browsing contents, then verify the example clearly describes the required path configuration.

Written by the indexing model from the issue text.