Make it clearer in documentation that runtime APIs are asynchronous

We are receiving several bug reports arising from misunderstandings or inadequate information of timing patterns in runtime APIs. We should elaborate more clearly that these APIs are asynchronous without latency guarantees, and clients should connect to various signals to make sure that calls like SetAttribute, setdatainputvalue etc. work correctly. This info is in the documentation, but perhaps bit hard to find as most of it seems to be in API signal descriptions.

For example: a sequence where a presentation is loaded through setSource, and setAttribute is immediately called for an element in the presentation will fail as presentation is not yet initialized and ready. Instead, the API client should wait for presentationReady signal.

Same applies to slide changes, and probably to other locations.