Updated test case documentation

docs/test_cases/t20017.md

@@ -0,0 +1,44 @@
+# t20017 - Test case for combine_free_functions_into_file_participants option
+## Config
+```yaml
+compilation_database_dir: ..
+output_directory: puml
+diagrams:
+  t20017_sequence:
+    type: sequence
+    combine_free_functions_into_file_participants: true
+    relative_to: ../../tests/t20017
+    glob:
+      - ../../tests/t20017/t20017.cc
+    include:
+      namespaces:
+        - clanguml::t20017
+    using_namespace:
+      - clanguml::t20017
+    start_from:
+      - function: "clanguml::t20017::tmain()"
+```
+## Source code
+File t20017.cc
+```cpp
+#include "include/t20017_a.h"
+#include "include/t20017_b.h"
+
+namespace clanguml {
+namespace t20017 {
+int tmain() { return b2(a1(a2(a3(1, 2), b1(3, 4)), 5), 6); }
+}
+}
+```
+File t20017_b.cc
+```cpp
+#include "include/t20017_b.h"
+
+namespace clanguml {
+namespace t20017 {
+int b1(int x, int y) { return x - y; }
+}
+}
+```
+## Generated UML diagrams
+![t20017_sequence](./t20017_sequence.svg "Test case for combine_free_functions_into_file_participants option")