Add some explanation for the instance AZ field

Since it's pretty confusing to understand why an instance AZ can be different
from the host AZ the instance belongs to, and since that field is never
updated even after an instance is scheduled to an host, we think it's good
to add a comment in the DB model to explain that.

Change-Id: Id0914530cbacf4cf5c3319f1401bf07b3112935d

nova/db/sqlalchemy/models.py

@@ -284,6 +284,9 @@ class Instance(BASE, NovaBase):
     launched_at = Column(DateTime)
     terminated_at = Column(DateTime)
 
+    # This always refers to the availability_zone kwarg passed in /servers and
+    # provided as an API option, not at all related to the host AZ the instance
+    # belongs to.
     availability_zone = Column(String(255))
 
     # User editable field for display in user-facing UIs