MSeq and MRand documentation: give an example.

Suggested-by: Dai Weituo

src/MRand.mli

@@ -1 +1,13 @@
 include Utils.MonadPlus
+
+(**
+   We demand that [MRand.run e] returns an infinite sequence of solutions
+   sampled from [e], for example
+   {[
+     (let open MRand in sum [return 1; return 2; return 3])
+     |> MRand.run     (* infinite stream of random samples *)
+     |> Seq.take 10   (* take the first 10 elements *)
+     |> List.of_seq
+   ]}
+   could be the list [[1; 2; 1; 2; 1; 1; 1; 3; 3; 2]].
+*)

src/MSeq.mli

@@ -1 +1,13 @@
 include Utils.MonadPlus
+
+(**
+   We demand that [MSeq.run e] returns the finite list of solutions
+   for [e], for example
+   {[
+     (let open MSeq in sum [return 1; return 2; return 3])
+     |> MSeq.run    (* list of all solutions *)
+     |> List.of_seq
+   ]}
+   should be the list [[1; 2; 3]] -- or maybe the same elements in
+   some other order.
+*)