Add FB number in the comment

Author: Muxi Yan

src/objective-c/README-CFSTREAM.md

@@ -24,14 +24,14 @@ case of problem and you want to disable CFStream on iOS, you can set environment
 "grpc\_cfstream=0".
 
 ## Caveats
-It is known to us that the CFStream API has some bug which will cause gRPC's CFStream networking
-layer to stall occasionally. The issue mostly occur on MacOS systems (including iOS simulators on
-MacOS); iOS may be affected too but we have not seen issue there. gRPC provides a workaround to this
-problem with an alternative poller based on CFRunLoop. The poller can be enabled by setting
-environment variable `GRPC_CFSTREAM_RUN_LOOP=1`. Note that the poller is a client side only poller
-that does not support running a server on it. That means if an app opts in to the CFRunLoop-based
-poller, the app cannot host a gRPC server (gRPC Objective-C does not support running a server but
-other languages running on iOS do support it).
+It is known to us that the CFStream API has some bug (FB6162039) which will cause gRPC's CFStream
+networking layer to stall occasionally. The issue mostly occur on MacOS systems (including iOS
+simulators on MacOS); iOS may be affected too but we have not seen issue there. gRPC provides a
+workaround to this problem with an alternative poller based on CFRunLoop. The poller can be enabled
+by setting environment variable `GRPC_CFSTREAM_RUN_LOOP=1`. Note that the poller is a client side
+only poller that does not support running a server on it. That means if an app opts in to the
+CFRunLoop-based poller, the app cannot host a gRPC server (gRPC Objective-C does not support running
+a server but other languages running on iOS do support it).
 
 ## Notes