fix: rename `pybind11::module` to `pybind11::module_` (#2489)

Support C++20. For backwards compatibility, we provide an alias for the old name.
This change is necessary to easily avoid errors when a compiler thinks
`module` is used as a keyword.
diff --git a/docs/reference.rst b/docs/reference.rst
index 752dfed..e3a61af 100644
--- a/docs/reference.rst
+++ b/docs/reference.rst
@@ -46,7 +46,7 @@
 Convenience classes for specific Python types
 =============================================
 
-.. doxygenclass:: module
+.. doxygenclass:: module_
     :members:
 
 .. doxygengroup:: pytypes