Extend Thing Link use cases in the Explainer

[farshidtz]

I think directory federation was never the intention for introducing Thing Links. It rather to allow referencing large TDs (from a small device to a powerful server) and dynamic TDs (from a directory to a device at the edge).
We recently added statements in Architecture and Discovery Process suggesting the use of Think Link for linking directories.

Originally posted by @farshidtz in #309 (comment)