Docs on module system should cite papers by Jacek Chrząszcz etc.

[Blaisorblade]

Description of the problem

@robbertkrebbers were discussing whether the Coq's module system design is documented by academic literature; no citations are given in the manual, but it seems such papers exist (https://coq.zulipchat.com/#narrow/stream/237977-Coq-users/topic/ANSSI.20and.20experimental.20SProp/near/247459612) and might be somewhat relevant to the current system.

[herbelin]

The docs on the module system were first written by Jacek Chrząszcz iirc. That would indeed be good to add a reference to his papers, as well as a reference to Judicaël Courant and Élie Soubiran's PhD theses. Do you think you'd be willing to propose a PR?