I *think* that field is a dinosaur. I cannot recall ever seeing it populated. Though it might populate for active alerts (I don't have any active in this environment to verify right now)
As for your second question:
UPDATE AlertStatus SET AlertMessage = "Your Message Here" WHERE AlertDefID = 'AlertDefID-Here'
With the caveat that without knowing what this field is related to in the overall schema, changing this could have unintended results. (i.e., BACK UP YOUR DB)
Seems to me that one should be, if not in fact already able to populate this field from any given alert definition (created via the typical alert creation process). Unfortunately I'm still trying to get this figured out myself.