qapi: Document type-safety considerations

Go into more details about the various types of valid expressions
in a qapi schema, including tweaks to document fixes being done
later in the current patch series.  Also fix some stale and missing
documentation in the QMP specification.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
