Moves `flags` field to top part of object description because
it is always included in the event.
If a field is present only for certain types of message updates,
the description begins by stating when the field is present:
"Only present if ...".
These fields are organized by the type of message update:
stream, stream and/or topic, topic, content.
If a field is not present due to a special event, the description
ends by stating when the field is not present:
"Not present if ...".
Adds documentation for fields currently required to be returned
with any `update_message` event.