odoc: change Poly variant to carry optional jkinds per variable
Poly of string list * t --> Poly of (string * string option) list * t
Each universally quantified variable can now carry an optional jkind
annotation (e.g. value_or_null, float64). This enables showing the
jkind at the binding site rather than repeating it at every use.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>