Updated test case documentation

docs/test_cases/t20028.md

@@ -0,0 +1,58 @@
+# t20028 - Conditional (ternary) '?:' operator test case
+## Config
+```yaml
+compilation_database_dir: ..
+output_directory: puml
+diagrams:
+  t20028_sequence:
+    type: sequence
+    glob:
+      - ../../tests/t20028/t20028.cc
+    include:
+      namespaces:
+        - clanguml::t20028
+    exclude:
+      namespaces:
+        - clanguml::t20028::detail
+    using_namespace:
+      - clanguml::t20028
+    start_from:
+      - function: "clanguml::t20028::tmain()"
+```
+## Source code
+File t20028.cc
+```cpp
+namespace clanguml {
+namespace t20028 {
+
+struct A {
+    int a() { return 0; }
+    int b() { return 1; }
+    int c() { return 2; }
+    int d() { return 3; }
+};
+
+namespace detail {
+struct B {
+    int e() { return 4; }
+};
+} // namespace detail
+
+int tmain()
+{
+    A a;
+    detail::B b;
+
+    int result{};
+
+    result = b.e() ? b.e() : 12;
+
+    result += a.a() ? a.b() + a.c() : a.d();
+
+    return result;
+}
+}
+}
+```
+## Generated UML diagrams
+![t20028_sequence](./t20028_sequence.svg "Conditional (ternary) '?:' operator test case")