add overloaded specs to eqwalizer stats

Summary:
- there is an interest to see a data about overloaded spec
- extending eqwalizer_stats with it

Reviewed By: VLanvin

Differential Revision: D60454935

fbshipit-source-id: 0a87eba233ec8c3653221149bfa57d689672bde7

docs/reference/stats.md

@@ -18,3 +18,8 @@ This indicates the suppression of all eqWAlizer errors associated to a function,
 using a `-eqwalizer({nowarn_function, foo/n})` pragma.
 eqWAlizer will not run on such a function, thus the use of this pragma is highly
 discouraged.
+
+### overloaded_spec
+
+This indicates an overloaded function spec (a spec with multiple clauses).
+The use of overloaded specs is discouraged.