Clarify relationship between SingleSelect, Dropdown and native select.

[danielck]

The Problem

Both Dropdown and SingleSelect appear to solve the same problem of selecting a single option from a dropdown list. At least on the surface, the only difference is that the SingleSelect is actually a combobox with autocomplete. The SingleSelect not working with VoiceOver on Safari/macOS is a pretty big issue.

Proposed Solution

Discourage use of SingleSelect in documentation until Safari with VoiceOver/macOS is fixed. Encouraging the use of a native select variant of this component or Dropdown would also help.