[EuiInMemoryTable] Needs to document its itemId prop

[chandlerprall]

This has confused me and at least 2 other devs. EuiInMemoryTable requires a selection prop and an itemId prop to enable selection. The only way to know this is looking at the Demo JS tab, spotting the itemId line, and recognizing it is important. Without itemId selection looks like it should be working but it mysteriously doesn't. Information about the necessary props should be included directly in the page's copy.