Greater standardization of sizing terminology

In class names (for greater brevity and to align better with the responsive utilities):
* `large` => `lg`
* `small` => `sm`
* `mini` => `xs` ("Extra small")

In screen size categories:
* "Tiny" => "Extra small"

Also makes some docs fixes regarding `.btn-xs` (formerly `.btn-mini`).

Doesn't fix issue №8926.