Updated test cases documentation

docs/test_cases/t20038.md

@@ -18,7 +18,7 @@ diagrams:
     using_namespace: clanguml::t20038
 ```
 ## Source code
-File t20038.cc
+File `tests/t20038/t20038.cc`
 ```cpp
 #include <future>
 
@@ -112,6 +112,23 @@ int tmain()
 }
 }
 ```
+File `tests/t20038/include/t20038.h`
+```cpp
+#pragma once
+
+namespace clanguml {
+namespace t20038 {
+
+template <typename T> T add_impl(T a, T b) { return a + b; };
+
+template <typename T> T add(T a, T b)
+{
+    // Invoke 'add' implementation
+    return add_impl(a, b);
+};
+}
+}
+```
 ## Generated PlantUML diagrams
 ![t20038_sequence](./t20038_sequence.svg "Sequence diagram comment decorator test case")
 ## Generated Mermaid diagrams