Add documentation for templated tests

docs/test-fixtures.md

@@ -30,6 +30,36 @@ class UniqueTestsFixture {
 
 The two test cases here will create uniquely-named derived classes of UniqueTestsFixture and thus can access the `getID()` protected method and `conn` member variables. This ensures that both the test cases are able to create a DBConnection using the same method (DRY principle) and that any ID's created are unique such that the order that tests are executed does not matter.
 
+
+Catch2 also provides `TEMPLATE_TEST_CASE_METHOD` that can be used together
+with templated fixtures to perform tests for multiple different types.
+However, unlike `TEST_CASE_METHOD`, `TEMPLATE_TEST_CASE_METHOD` requires
+the tag specification to be non-empty, as it is followed by further macros
+arguments.
+
+Also note that, because of limitations of the C++ preprocessor, if you
+want to specify a type with multiple template parameters, you need to
+enclose it in parentheses, e.g. `std::map<int, std::string>` needs to be
+passed as `(std::map<int, std::string>)`.
+
+Example:
+```cpp
+template< typename T >
+struct Template_Fixture {
+    Template_Fixture(): m_a(1) {}
+
+    T m_a;
+};
+
+TEMPLATE_TEST_CASE_METHOD(Template_Fixture,"A TEMPLATE_TEST_CASE_METHOD based test run that succeeds", "[class][template]", int, float, double) {
+    REQUIRE( Template_Fixture<TestType>::m_a == 1 );
+}
+```
+
+_While there is an upper limit on the number of types you can specify
+in single `TEMPLATE_TEST_CASE`, the limit is very high and should not
+be encountered in practice._
+
 ---
 
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