doc: Bluetooth: fix "Initialization" section

The Initialization section contains two errors:

1. bt_enable() is the public API for initializing the stack, not
   bt_init().

2. When bt_enable() is passed a function pointer, initialization
   happens asynchronously, not synchronously.

Fix these.

Change-Id: If02a6d6fe82f546b5cb6f9fdf76c6641baa6cac8
Signed-off-by: Marti Bolivar <marti.bolivar@linaro.org>