add a paragraph in the manual to explain equations on non-local types
add a paragraph in the manual to explain equations on non-local types

eliberis/ocaml,yunxing/ocaml,eliberis/ocaml,yunxing/ocaml,eliberis/ocaml,msprotz/ocaml,eliberis/ocaml,gerdstolpmann/ocaml,msprotz/ocaml,gerdstolpmann/ocaml,msprotz/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,msprotz/ocaml,msprotz/ocaml,yunxing/ocaml,yunxing/ocaml,yunxing/ocaml,eliberis/ocaml