Updated test cases documentation

docs/test_cases/t00056.md

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+# t00056 - Basic C++ concepts test case
+## Config
+```yaml
+compilation_database_dir: ..
+output_directory: puml
+diagrams:
+  t00056_class:
+    type: class
+    glob:
+      - ../../tests/t00056/t00056.cc
+    include:
+      namespaces:
+        - clanguml::t00056
+    using_namespace:
+      - clanguml::t00056
+```
+## Source code
+File t00056.cc
+```cpp
+#include <string>
+
+namespace clanguml {
+namespace t00056 {
+
+template <typename T, typename L>
+concept greater_than_simple = sizeof(T) > sizeof(L);
+
+template <typename T, typename P>
+concept greater_than_with_requires = requires(T l, P r)
+{
+    sizeof(l) > sizeof(r);
+};
+
+// Constraint expression
+template <typename T>
+concept max_four_bytes = sizeof(T) <= 4;
+
+// Simple requirement
+template <typename T>
+concept iterable = requires(T container)
+{
+    container.begin();
+    container.end();
+};
+
+// Type requirement
+template <typename T>
+concept has_value_type = requires
+{
+    typename T::value_type;
+};
+
+template <typename T>
+concept convertible_to_string = requires(T s)
+{
+    std::string{s};
+};
+
+// Compound requirement
+// ...
+
+// Combined concept
+template <typename T>
+concept iterable_with_value_type = iterable<T> && has_value_type<T>;
+
+template <typename T>
+concept iterable_or_small_value_type =
+    iterable_with_value_type<T> || max_four_bytes<T>;
+
+// Simple type constraint
+template <max_four_bytes T> struct A {
+    T a;
+};
+
+// Requires constant expression
+template <typename T>
+requires iterable_or_small_value_type<T>
+struct B {
+    T b;
+};
+
+// Anonymous concept requirement (TODO)
+template <convertible_to_string T>
+requires requires(T t)
+{
+    --t;
+    t--;
+}
+struct C {
+    T c;
+};
+
+template <iterable T1, typename T2, iterable T3, typename T4, typename T5>
+requires max_four_bytes<T2> && max_four_bytes<T5>
+struct D {
+};
+
+template <typename T1, typename T2, typename T3>
+requires greater_than_with_requires<T1, T3>
+struct E {
+    T1 e1;
+    T2 e2;
+    T3 e3;
+};
+
+template <typename T1, typename T2, typename T3>
+requires greater_than_simple<T1, T3>
+struct F {
+    T1 f1;
+    T2 f2;
+    T3 f3;
+};
+}
+}
+```
+## Generated UML diagrams
+![t00056_class](./t00056_class.svg "Basic C++ concepts test case")