Add a tooltip to the memory dropdown ("data source" / "call tree summary strategy")

For the options like "Deallocated memory" + "Deallocation sites", it's not easy to find out what they mean.

We have a tooltip though! It's on the ``. But you only see the tooltip while the `` menu is open.

I think we should add the description of the selected option to the `` itself too, so that you get a tooltip when the menu is closed.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-873)