DOCSP-50133: Update 4.1 breaking changes (#133)

Author: norareidy

source/upgrade.txt

@@ -133,7 +133,12 @@ Build System Breaking Changes in v4.1
 
 The v4.1 driver introduces the following breaking changes to the build system:
 
-- Requires MongoDB C Driver v2.0.0 or later.
+- Requires MongoDB C Driver v2.0.0 or later
+
+ABI Breaking Changes in v4.1
+````````````````````````````
+The v4.1 driver introduces the following breaking changes to the ABI:
+
 - Declares C++17 polyfill implementations for ``optional<T>`` and ``string_view`` in the ``bsoncxx::v1::stdx`` namespace. The
   ``bsoncxx::v_noabi::stdx`` namespace now contains type aliases to their v1
   equivalents.