Update docs, add HANDLED, NOT_HANDLED and getCurrentMessage.

The EBNF statemachine description language was to difficult
to remember changed to use a simpler and more obvious psuedo
language.

Added HANDLED and NOT_HANDLED as it makes the psuedo code
more obvious.

Added getCurrentMessage primarily for use by code in enter
so that it can know why the new state is being entered.

Change-Id: I1446e417b77684fbde0020b1da0975eedc57cce4