Added note about type unions in generic types

docs/manual.md

@@ -170,6 +170,28 @@ When using automatic types, peon will behave similarly to C++ (think: templates)
 typecheck and compile the function once it is called with a given type signature. For this reason,
 automatic and parametrically polymorphic types cannot be used together in peon code.
 
+Another noteworthy concept to keep in mind is that of type unions. For example, take this snippet:
+
+```
+fn foo(x: int32): int32 {
+    return x;
+}
+
+
+fn foo(x: int): int {
+    return x;
+}
+
+
+fn identity[T: int | int32](x: T): T {
+    return foo(x);
+}
+```
+
+This code will, again, fail to compile: this is because as far as peon is concerned, `foo` is not
+defined for both `int` and `int32` _at the same time_. In order for that to work, `foo` would need
+to be rewritten with `T: int32 | int` as its generic argument type in order to avoid the ambiguity.
+
 
 #### More generics