[DOC BLD FIX] Fix docstring formatting in message_field
There was an improperly indented section in
cinder.message.message_field that was causing a sphinx warning
to be generated.
I fixed the formatting to avoid this issue.
Use Resource, Action and Detail's combination to indicate the Event
in the format of:
+
EVENT: VOLUME_RESOURCE_ACTION_DETAIL
+
Also, use exception-to-detail mapping to decrease the workload of
classifying event in cinder's task code.
"""
