Update the process.entity_id description with recommended generation methods.

These methods will allow entity_id to be generated reproducibly, while being
unique to a process.

If different data source collectors use the recommeded generation method, and
observe events from the same process, they will generate the same entity_id, and
it will be possible to corralate the  events and identify them as belonging to the
same process later.

Re-order the entity_id fields to have the PID, start time first. To
improve human readability and scanning effeciency, the fields that
change most often are first.

Also redefined Windows entity_id to use existing Windows process fields
that are more likely to be unique for the process.