lttng-enable-event(1): update the Filter expression section

lttng-enable-event(1): update the Filter expression section

This patch documents new features of filter expressions:

* Bracket and dot notations to get nested fields.
* New bitwise operators.
* Custom operator precedence table.
* Formal rules for how integers are represented for the sake of
  the evaluation (casted to signed/unsigned 64-bit integers depending
  on the operator).

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>