Update, expand, and document plugin tutorial 2

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diff --git a/doc/plugin_tutorial/tuto2/src/counter.ml b/doc/plugin_tutorial/tuto2/src/counter.ml
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+(*
+ * This file defines our counter, which we use in the Count command.
+ *)
+
+(*
+ * Our counter is simply a reference called "counter" to an integer.
+ *
+ * Summary.ref behaves like ref, but also registers a summary to Coq.
+ *)
+let counter = Summary.ref ~name:"counter" 0
+
+(*
+ * We can increment our counter:
+ *)
+let increment () =
+  counter := succ !counter
+
+(*
+ * We can also read the value of our counter:
+ *)
+let value () =
+  !counter