Clarify api docs

[LeeLenaleee]

Comming from #10329 (reply in thread) and #10390

Add some more methods to the API page of the docs for clarification and add sample of how to pass the update mode to updating charts page

[stockiNail]

@LeeLenaleee if I may, let me jump in this topic. I see that draw method is not documented. I'm wondering why.

And also isDatasetVisible(index). Maybe there is a good reason and apologize if I'm wrong.

[LeeLenaleee]

No I am just verry tired and was planning to get at least the getSortedVisibleDatasetMetas documented and on the update page clarify the mode with a sample and while I was doing that I added some more that I quickly noticed that also missed and could be added

[stockiNail]

Thank you @LeeLenaleee !! I have this item in my long TODO list and as soon as I have time, maybe I can submit a PR adding those 2 methods (even if my English is not good enough to write a good documentation ;) ).