chore(sdk-crashes): Info on custom stacktrace rule

Add comment on custom stack trace rule.

src/sentry/utils/sdk_crashes/event_stripper.py

@@ -164,6 +164,9 @@ def _strip_frames(
     """
 
     def strip_frame(frame: MutableMapping[str, Any]) -> MutableMapping[str, Any]:
+        # Set in_app to True for SDK frames for grouping. Anyways the grouping config will set in_app false
+        # for all Cocoa SDK frames. To not change the grouping logic, we must add the following stacktrace
+        # rule  `stack.abs_path:Sentry.framework +app` to the project with the ID SDK_CRASH_DETECTION_PROJECT_ID.
         if sdk_crash_detector.is_sdk_frame(frame):
             frame["in_app"] = True