Some changes based on feedback

* weakened: "the LRS SHOULD replace its internal representation of that activity definition with the loaded definition" to take into account statements possibly having more information/translations
* removed The LRS MAY change its internal representation... since it's redundant
* in general identifier metadata section, clarified activity definitions have their own format
* clarified that metadata SHOULD be available only for URLs coined for the purpose of use in this specification (in order to avoid the implication that URLs without metadata should be avoided)
* In general identifier metadata section, clarified that metadata may come from other sources or formats as well, particularly translations