qapi2texi: Generate descriptions for simple union tags

Simple union tags carry no type information, because their type is
implicit.  Their description should make up for it, but many have
none.  Generate one automatically then.

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

  -- Simple Union: ImageInfoSpecific

      A discriminated record of image format specific information
      structures.

      Members:
      'type'
-          Not documented
+          One of "qcow2", "vmdk", "luks"
      'data: ImageInfoSpecificQCow2' when 'type' is "qcow2"
      'data: ImageInfoSpecificVmdk' when 'type' is "vmdk"
      'data: QCryptoBlockInfoLUKS' when 'type' is "luks"

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1489582656-31133-29-git-send-email-armbru@redhat.com>