Fix improper documentation on casting non_exhaustive enums

src/attributes/type_system.md

@@ -163,12 +163,33 @@ match message {
 }
 ```
 
-It's also not allowed to cast non-exhaustive types from foreign crates.
+However, casting non-exhaustive types from foreign crates is generally disallowed, except when dealing with enums that have no non-exhaustive variants.
+
+For example, the following enum can be cast because it doesn't contain any non-exhaustive variants:
+```rust, ignore
+#[non_exhaustive]
+pub enum Example {
+    First,
+    Second
+}
+```
+
+However, if the enum contains even a single non-exhaustive variant, casting will result in an error. Consider this modified version of the same enum:
+
+```rust, ignore
+#[non_exhaustive]
+pub enum Example {
+    First,
+    #[non_exhaustive]
+    Second
+}
+```
+
 ```rust, ignore
-use othercrate::NonExhaustiveEnum;
+use othercrate::EnumWithNonExhaustiveEnumVariants;
 
-// Cannot cast a non-exhaustive enum outside of its defining crate.
-let _ = NonExhaustiveEnum::default() as u8;
+// Error: cannot cast an enum with a non-exhaustive variant when it's defined in another crate
+let _ = EnumWithNonExhaustiveEnumVariants::default() as u8;
 ```
 
 Non-exhaustive types are always considered inhabited in downstream crates.