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>
---
 scripts/qapi2texi.py | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/scripts/qapi2texi.py b/scripts/qapi2texi.py
index ab6b6cd..282adf4 100755
--- a/scripts/qapi2texi.py
+++ b/scripts/qapi2texi.py
@@ -148,11 +148,16 @@ def texi_members(doc, what, base, variants, member_func):
     """Format the table of members"""
     items = ''
     for section in doc.args.itervalues():
+        # TODO Drop fallbacks when undocumented members are outlawed
         if section.content:
-            desc = str(section)
+            desc = texi_format(str(section))
+        elif (variants and variants.tag_member == section.member
+              and not section.member.type.doc_type()):
+            values = section.member.type.member_names()
+            desc = 'One of ' + ', '.join(['@t{"%s"}' % v for v in values])
         else:
             desc = 'Not documented'
-        items += member_func(section.member) + texi_format(desc) + '\n'
+        items += member_func(section.member) + desc + '\n'
     if base:
         items += '@item The members of @code{%s}\n' % base.doc_type()
     if variants:
-- 
2.7.4