Updated test case documentation

docs/test_cases/t20004.md

@@ -0,0 +1,41 @@
+# t20004 - Function template instantiation sequence diagram test case
+## Config
+```yaml
+compilation_database_dir: ..
+output_directory: puml
+diagrams:
+  t20004_sequence:
+    type: sequence
+    glob:
+      - ../../tests/t20004/t20004.cc
+    include:
+      namespaces:
+        - clanguml::t20004
+    using_namespace:
+      - clanguml::t20004
+    start_from:
+      - function: "clanguml::t20004::main()"
+```
+## Source code
+File t20004.cc
+```cpp
+namespace clanguml {
+namespace t20004 {
+
+template <typename T> T m4(T p);
+
+template <typename T> T m3(T p) { return m4<T>(p); }
+
+template <typename T> T m2(T p) { return m3<T>(p); }
+
+template <typename T> T m1(T p) { return m2<T>(p); }
+
+template <> [[maybe_unused]] int m4<int>(int p) { return p + 2; }
+
+int main() { return m1<int>(0); }
+}
+}
+
+```
+## Generated UML diagrams
+![t20004_sequence](./t20004_sequence.svg "Function template instantiation sequence diagram test case")