Improve macro type handling for types with commas

- PYBIND11_MAKE_OPAQUE now takes ... rather than a single argument and
  expands it with __VA_ARGS__; this lets templated, comma-containing
  types get through correctly.
- Adds a new macro PYBIND11_TYPE() that lets you pass the type into a
  macro as a single argument, such as:

      PYBIND11_OVERLOAD(PYBIND11_TYPE(R<1,2>), PYBIND11_TYPE(C<3,4>), func)

  Unfortunately this only works for one macro call: to forward the
  argument on to the next macro call (without the processor breaking it
  up again) requires also adding the PYBIND11_TYPE(...) to type macro
  arguments in the PYBIND11_OVERLOAD_... macro chain.
- updated the documentation with these two changes, and use them at a couple
  places in the test suite to test that they work.

tests/test_virtual_functions.cpp

@@ -207,7 +207,9 @@ TEST_SUBMODULE(virtual_functions, m) {
 
         void f() override {
             py::print("PyA.f()");
-            PYBIND11_OVERLOAD(void, A, f);
+            // This convolution just gives a `void`, but tests that PYBIND11_TYPE() works to protect
+            // a type containing a ,
+            PYBIND11_OVERLOAD(PYBIND11_TYPE(typename std::enable_if<true, void>::type), A, f);
         }
     };