Mojo: Move comment (per TODO).

mojo/system/core_impl.cc

Index: mojo/system/core_impl.cc
diff --git a/mojo/system/core_impl.cc b/mojo/system/core_impl.cc
index 859617ae14d7b7056421cfd2b1a159a2d2151397..7477f9142a06f609b4ac44206965bc1595a7987a 100644
--- a/mojo/system/core_impl.cc
+++ b/mojo/system/core_impl.cc
@@ -186,6 +186,13 @@ MojoResult CoreImpl::CreateMessagePipe(MojoHandle* message_pipe_handle0,
   return MOJO_RESULT_OK;
 }
 
+// Implementation note: To properly cancel waiters and avoid other races, this
+// does not transfer dispatchers from one handle to another, even when sending a
+// message in-process. Instead, it must transfer the "contents" of the
+// dispatcher to a new dispatcher, and then close the old dispatcher. If this
+// isn't done, in the in-process case, calls on the old handle may complete
+// after the the message has been received and a new handle created (and
+// possibly even after calls have been made on the new handle).
 MojoResult CoreImpl::WriteMessage(MojoHandle message_pipe_handle,
                                   const void* bytes,
                                   uint32_t num_bytes,