Add explicit aria-label to placeholder-only inputs

While `placeholder` is nominally valid per spec
http://rawgit.com/w3c/html-api-map/master/index.html#accessible-name-and-description-calculation
it is inelegant, a fallback, and not supported in ios/safari/voiceover
and android/chrome/talkback, to name a few combinations

`aria-describedby` is also not really the right tool for providing a
name/label equivalent, so remove that from the list of possible
alternative methods / clarify how it can be used