A flat union's branch brings in the members of another type.  Generate
a suitable reference to that type.

Example change (qemu-qmp-ref.txt):

  -- Flat Union: QCryptoBlockOpenOptions

      The options that are available for all encryption formats when
      opening an existing volume

      Members:
      The members of 'QCryptoBlockOptionsBase'
+     The members of 'QCryptoBlockOptionsQCow' when 'format' is "qcow"
+     The members of 'QCryptoBlockOptionsLUKS' when 'format' is "luks"

      Since: 2.6

A simple union's branch adds a member 'data' of some other type.
Generate documentation for that member.

Example change (qemu-qmp-ref.txt):

  -- Simple Union: SocketAddress

      Captures the address of a socket, which could also be a named file
      descriptor

      Members:
      'type'
	   Not documented
+     'data: InetSocketAddress' when 'type' is "inet"
+     'data: UnixSocketAddress' when 'type' is "unix"
+     'data: VsockSocketAddress' when 'type' is "vsock"
+     'data: String' when 'type' is "fd"

      Since: 1.3
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Message-Id: <1489582656-31133-28-git-send-email-armbru@redhat.com>