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: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1489582656-31133-23-git-send-email-armbru@redhat.com>
