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: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Message-Id: <1489582656-31133-29-git-send-email-armbru@redhat.com>

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: