Doc note—main docs should discusses selected item label.

[arthurevans]

If you're using two-line items, for example, it may be desirable to set an explicit label property on the items.

This is documented, obliquely, under selectedItemLabel, but based on user feedback, I think it should be called out explicitly in the main body of the doc—something like this:

The text shown on the closed paper-dropdown-menu is set to the paper-dropdown-menu's label property, if no item has been selected. Once an item has been selected, it displays the item's label property or attribute, or—if the item has no label—the item's text content, trimmed of leading and trailing whitespace.