Avoid C-style casts for pointers in docs (#2487)

Why only for pointers? Because C casts are hard to grep for.
diff --git a/docs/advanced/functions.rst b/docs/advanced/functions.rst
index 2814adf..c895517 100644
--- a/docs/advanced/functions.rst
+++ b/docs/advanced/functions.rst
@@ -360,7 +360,7 @@
 .. code-block:: cpp
 
     py::class_<MyClass>("MyClass")
-        .def("myFunction", py::arg("arg") = (SomeType *) nullptr);
+        .def("myFunction", py::arg("arg") = static_cast<SomeType *>(nullptr));
 
 Keyword-only arguments
 ======================