Merge "init: cleanup exec_service documentation" am: 61559e49

  Init halts executing commands until the forked process exits.

`exec_start <service>`

> Start service a given service and halt processing of additional init commands

  until it returns.  It functions similarly to the `exec` command, but uses an

  existing service definition instead of providing them as arguments.

> Start a given service and halt the processing of additional init commands

  until it returns.  The command functions similarly to the `exec` command,

  but uses an existing service definition in place of the exec argument vector.

`export <name> <value>`

> Set the environment variable _name_ equal to _value_ in the