init.rc: Add documentation

Android developers should never place files in /data/local/tmp.
Files or directories in /data/local/tmp can be minipulated by the
shell user.

Android developers should never create world-writable files
or directories. This is a common source of security vulnerabilities.

Change-Id: I6d2cd620ab49d8ca3f39282f7d2ed682a9ba91c3