[luatex] glue fill order

[David Carlisle]

Hironobu Yamashita kindly tells me that this comes from Omega and is
documented there, also that e-ptex has fi glue as well but in that
case they keep \gluestretchorder compatible with e-tex.

https://chat.stackexchange.com/transcript/message/51983994#51983994

So I suppose this is a request for the \gluestretchorder difference to
be documented in the manual since the only mention of
\gluestretchorder in the luatex manual is in the list of etex
commands.

David