CXX-2723 Explicitly document throwing exception from APM callback is undefined behavior (#1015)

* CXX-2723 Document exception undefined behavior for APM callbacks

* Upgrade Doxyfile to 1.9.1

* Call `std::terminate()` if an APM callback exits via an exception

Author: eramongodb

CHANGELOG.md

@@ -16,3 +16,6 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 - Do not build tests as part of `all` target. Configure with `BUILD_TESTING=ON` to build tests.
+
+### Fixed
+- Explicitly document that throwing an exception from an APM callback is undefined behavior.