[docs] Add more detail to tooltip vs. popover guidelines

[hellogreg]

Based on some side chats with @heyMP, we think it could be helpful to expand a bit on the differences between (and use cases for) tooltip and popover, as currently written on the tooltip guidelines page.

Perhaps a table with the following info would be helpful--or just keep them in lists like this!

Tooltip

1. Appears when triggering element receives mouse hover, keyboard focus, or is tapped.
2. Contains only plain text.
3. Has no internal means of dismissal (e.g., a close button).
4. Dismissed when triggering element loses hover/focus, or when a touchscreen user taps elsewhere.

Popover

1. Appears when triggering button is clicked.
2. Can contain headings, images, and interactive content (e.g., links).
3. Has a close button.
4. Dismissed when close button is clicked, user presses esc, or user clicks/taps outside the popover.

We may also want to ask implementors to investigate whether a toolitp/popover is appropriate at all. If info is helpful enough that it warrants including in a tooltip, why is it being hidden by default? Not saying tooltips are bad! Just saying we should always be mindful about why we're using them.