[charts] Document how to freez the tooltip

[alexfauquette]

I’m also exploring how to make the tooltip controllable. At first, I assumed the tooltip would automatically sync with the highlighted item’s state. I laid out my use case in detail here: #20187

In short: what I’m trying to do is allow the tooltip to persist after a click—so that users can interact with it (hover, click links/buttons inside, etc.).
Adding this comment since I think its another use case where having a controlled tooltip would be very useful.

Originally posted by @nizans in #14455

[linear]

X-4640