Merge "Update docs around capabilities and root processes"

  capability without the "CAP\_" prefix, like "NET\_ADMIN" or "SETPCAP". See

  http://man7.org/linux/man-pages/man7/capabilities.7.html for a list of Linux

  capabilities.

  If no capabilities are provided, then all capabilities are removed from this service, even if it

  runs as root.

  If no capabilities are provided, then behaviour depends on the user the service runs under:

    * if it's root, then the service will run with all the capabitilies (note: whether the

        service can actually use them is controlled by selinux);

    * otherwise all capabilities will be dropped.

`class <name> [ <name>\* ]`

> Specify class names for the service.  All services in a

  using this new mechanism, processes can use the user option to

  select their desired uid without ever running as root.

  As of Android O, processes can also request capabilities directly in their .rc

  files. See the "capabilities" option below.

  files. See the "capabilities" option above.

`writepid <file> [ <file>\* ]`

> Write the child's pid to the given files when it forks. Meant for