DOCSP-45313: Remove mongocxx::stdx namespace (#83)

* DOCSP-45313: Remove mongocxx::stdx namespace

* MW feedback

* EC feedback

source/upgrade.txt

@@ -159,6 +159,12 @@ The v4.0 driver introduces the following breaking changes to the API:
   library equivalents. For context, see the :ref:`Build System Breaking Changes
   <version-4.0-build-system-breaking>` section.
 
+- Removes the ``mongocxx::stdx`` namespace. Replace the following
+  types in your code:
+
+  - ``mongocxx::stdx::optional<T>``. Instead, use ``bsoncxx::stdx::optional<T>``.
+  - ``mongocxx::stdx::string_view``. Instead, use ``bsoncxx::stdx::string_view``.
+
 - Removes the following undocumented ``using`` directives and declarations:
 
   - ``bsoncxx::builder::types`` from the ``<bsoncxx/builder/list.hpp>`` header file.