Merge "Point to exec_start from start in init documentation"

  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.

  See the `exec_start` command for a synchronous version of `start`.

> This creates an important consequence that if the service offers

  functionality to other services, such as providing a