Added note about type parameters in ifaces.

Specficially, the type parameters should be left off of the function
signatures in both the iface and the impl if they are already in the
iface declaration.

Author: Havvy

doc/tutorial.md

@@ -2504,6 +2504,10 @@ needed because it could also, for example, specify an implementation
 of `seq<int>`—the `of` clause *refers* to a type, rather than defining
 one.
 
+Note that functions do not explicitly have the type parameters that 
+are provided by the iface. It will cause a compile-time error if you
+include them in the iface or impl.
+
 ## Casting to an interface type
 
 The above allows us to define functions that polymorphically act on