Update comment to clarify purpose of removing app from WeakSet

Author: artemziborev

instrumentation/opentelemetry-instrumentation-fastapi/src/opentelemetry/instrumentation/fastapi/__init__.py

@@ -359,7 +359,8 @@ def uninstrument_app(app: fastapi.FastAPI):
         app.middleware_stack = app.build_middleware_stack()
         app._is_instrumented_by_opentelemetry = False
 
-        # Remove the app from the set of instrumented apps to prevent memory leaks
+        # Remove the app from the set of instrumented apps to avoid calling uninstrument twice
+        # if the instrumentation is later disabled or such
         # Use discard to avoid KeyError if already GC'ed
         _InstrumentedFastAPI._instrumented_fastapi_apps.discard(app)