Sometimes we use the 'struct' keyword in headers to help us
reduce dependencies between header files.  Document that
practice.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>