terminology already in used).

informations about the subentries: we remember whether we are at any
level, at the next level or at the self level.

Incidentally, we also take into account option Printing Parentheses in
custom entries.

This is a more principled fix to coq#12775/coq#13018 than coq#13073 (printing
bugs in custom entry rules with no explicit level).

Compared to "subentries", the name "entry_relative" insists on the
syntactic structure of the type rather than on its semantic origin,
since, after all, subentries are promoted as entries in the recursive
printing loop.

…ther doc.

We hard-wire a coercion in constr from level 0 to the top level.