init: guarantee the ordering that actions are executed in

The README.md states that this ordering is not guaranteed to give
flexibility for the future, however it's time to state that this
ordering is guaranteed, especially since:

1) We have a tests, EventTriggerOrder and
   EventTriggerOrderMultipleFiles, which have guaranteed this ordering
   since 2017.
2) We have users requesting and depending on this order

Also update some slightly out of date parts of the documentation:
1) We import /system/etc/init/hw/init.rc instead of /init.rc as the
   first import
2) We additionally import /system_ext/etc/init and /product/etc/init

Test: n/a
Change-Id: I6d7b8d9e52f0d52bee320d5074ebb74a537f9150