Document property generator security implications

[lanthaler]

We discussed this as part of #160, property generators might be used for DoS attacks as an expanded document might be considerably larger than the input document. We should also document that for expansion in general.

[lanthaler]

I just added more information to the security considerations section mentioning that a JSON-LD document might expand enourmously when being processed. I think that now everything we need to mention is there.

We may wanna discuss briefly whether we should add a flag to JsonLdOptions to disable property generators (e.g., to just use the first IRI of a property generator). I will probably add such a flag to my processor but it's not really necessary we standardize it. I thought it might be worth bringing it up nevertheless.