Rewrite the description about triggers

Triggers

--------

Triggers are strings which can be used to match certain kinds of

events and used to cause an action to occur.

Triggers of an action specifies one or more conditions when satisfied

execute the commands in the action. A trigger encodes a single atomic

condition, and multiple triggers can be combined using the `&&`

operator to form a bigger AND condition.

Triggers are subdivided into event triggers and property triggers.

There are two types of triggers: event triggers and action triggers.

An action can have multiple property triggers but may have only one

event trigger.

Event triggers are strings triggered by the 'trigger' command or by

the QueueEventTrigger() function within the init executable.  These

take the form of a simple string such as 'boot' or 'late-init'.

An event trigger takes the simple form of `<event>` where `<event>` is

the name of a boot stage like `early-init` or `boot`. This trigger

is satisfied when init reaches the stage via the `trigger` command or

by the `QueueEventTrigger()` function in the init executable.

Property triggers are strings triggered when a named property changes

value to a given new value or when a named property changes value to

any new value.  These take the form of 'property:<name>=<value>' and

'property:<name>=\*' respectively.  Property triggers are additionally

evaluated and triggered accordingly during the initial boot phase of

init.

A property trigger takes the form of `property:<name>=<value>`. This

trigger is satisfied when the property of name `<name>` is found to

have the value of `<value>` when the check is made. The `<value>` part

can be `\*` to match with any value.

An Action can have multiple property triggers but may only have one

event trigger.

The check for property trigger is made in the following cases:

For example:

`on boot && property:a=b` defines an action that is only executed when

the 'boot' event trigger happens and the property a equals b at the moment. This

will NOT be executed when the property a transitions to value b after the `boot`

event was triggered.

* All property triggers get checked at least once when the `boot`

  event is finished (i.e. when the last command under `on boot ...` is

finished).

* After the one-time check, `property:a=b` is checked when property `a`

  is newly created, or when the property is set to a new value.

* Property triggers are also checked when other triggers in the same

  action are checked. For example, `property:a=b && property:c=d` is

checked not only when property `a` gets a new value, but also when

property `c` gets a new value (and of course when the one-time check

is made).

* Before the one-time check, `property:a=b` without an event trigger

  is NOT checked, even if property `a` gets a new value. Care must be

taken since this is a non-intuitive behavior, which unfortunately

can't be changed due to compatibility concerns.

Some examples:

`on property:a=b` is executed in two cases:

`on property:a=b && property:c=d` defines an action that is executed

at three times:

1. during the one-time check if property `a` is `b` at the moment.

2. if property `a` is set to or changed to `b` after the one-time

   check, but not before then.

   1. During initial boot if property a=b and property c=d.

   2. Any time that property a transitions to value b, while property c already equals d.

   3. Any time that property c transitions to value d, while property a already equals b.

`on property:a=b && property:c=d` is executed in three cases:

Note that, for bootloader-provided properties (ro.boot.*), their action cannot be

auto-triggered until `boot` stage. If they need to be triggered earlier, like at `early-boot`

stage, they should be tied to the `event`. For example:

1. during the one-time check if property `a` is `b` and property `c`

   is `d` at the moment.

2. (after the one-time check) property `a` becomes `b` while property

   `c` already equals to `d`.

3. (after the one-time check) property `c` becomes `d` while property

   `a` already equals to `b`.

`on early-boot && property:a=b`.

`on property:a=b && post-fs` is executed in one case only:

1. `post-fs` is triggered while property `a` already equals to `b`.

   This is NOT executed when property `a` becomes `b` AFTER `post-fs`.

Trigger Sequence

----------------