Add documentation to Prec type (ocaml-ppx#1878)

Author: gpetiot

lib/Prec.mli

@@ -12,23 +12,23 @@
 (** Precedence levels of Ast terms. *)
 type t =
   | Low
-  | Semi
-  | LessMinus
-  | ColonEqual
-  | As
-  | Comma
-  | MinusGreater
-  | BarBar
-  | AmperAmper
-  | InfixOp0
-  | InfixOp1
-  | ColonColon
-  | InfixOp2
-  | InfixOp3
-  | InfixOp4
-  | UMinus
+  | Semi  (** [;] *)
+  | LessMinus  (** [<-] *)
+  | ColonEqual  (** [:=] *)
+  | As  (** [as] *)
+  | Comma  (** [,] *)
+  | MinusGreater  (** [->] *)
+  | BarBar  (** [or] and [||] *)
+  | AmperAmper  (** [&] and [&&] *)
+  | InfixOp0  (** [=…], [<…], [>…], [|…], [&…], [$…] and [!=] *)
+  | InfixOp1  (** [@…] and [^…] *)
+  | ColonColon  (** [::] *)
+  | InfixOp2  (** [+…] and [-…] *)
+  | InfixOp3  (** [*…], [/…], [%…], [mod], [land], [lor] and [lxor] *)
+  | InfixOp4  (** [**…], [lsl], [lsr] and [asr] *)
+  | UMinus  (** [-] and [-.] (prefix) *)
   | Apply
-  | HashOp
+  | HashOp  (** [#…] *)
   | Dot  (** [x.y] and [x#y] *)
   | High
   | Atomic