Add comments on what 'covering' a write means

src/core/lib/iomgr/tcp_posix.cc

@@ -204,6 +204,15 @@ static void drop_uncovered(grpc_tcp* tcp) {
   GPR_ASSERT(old_count != 1);
 }
 
+/* gRPC API considers a Write operation to be done the moment it clears ‘flow
+   control’ i.e., and not necessarily sent on the wire. This means that the
+   application MAY NOT call `grpc_completion_queue_next/pluck` in a timely
+   manner when its `Write()` API is acked.
+
+   We need to ensure that the fd is 'covered' (i.e being monitored by some
+   polling thread and progress is made) and hence add it to a backup poller
+   here */
+
 static void cover_self(grpc_tcp* tcp) {
   backup_poller* p;
   gpr_atm old_count =