qapi2texi: Explain enum value undocumentedness more clearly

Instead of not saying anything when we have no documentation, say "Not
documented".

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

  -- Enum: GuestPanicAction

      An enumeration of the actions taken when guest OS panic is detected

      Values:
      'pause'
           system pauses
      'poweroff'
+          Not documented

      Since: 2.1 (poweroff since 2.8)
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Message-Id: <1489582656-31133-23-git-send-email-armbru@redhat.com>

scripts/qapi2texi.py

@@ -145,7 +145,10 @@ def texi_members(doc, what, member_func, show_undocumented):
     for section in doc.args.itervalues():
         if not section.content and not show_undocumented:
             continue          # Undocumented TODO require doc and drop
-        desc = str(section)
+        if section.content:
+            desc = str(section)
+        else:
+            desc = 'Not documented'
         items += member_func(section.member) + texi_format(desc) + '\n'
     if not items:
         return ''