Merge "init: add warning that `start` is not synchronous"

`class_start <serviceclass>`

> Start all services of the specified class if they are

  not already running.

  not already running.  See the start entry for more information on

  starting services.

`class_stop <serviceclass>`

> Stop and disable all services of the specified class if they are

`start <service>`

> Start a service running if it is not already running.

  Note that this is _not_ synchronous, and even if it were, there is

  no guarantee that the operating system's scheduler will execute the

  service sufficiently to guarantee anything about the service's status.

> This creates an important consequence that if the service offers

  functionality to other services, such as providing a

  communication channel, simply starting this service before those

  services is _not_ sufficient to guarantee that the channel has

  been set up before those services ask for it.  There must be a

  separate mechanism to make any such guarantees.

`stop <service>`

> Stop a service from running if it is currently running.