[mojo-edk] Simplify multiprocess pipe bootstrap

mojo/edk/embedder/embedder.h

Index: mojo/edk/embedder/embedder.h
diff --git a/mojo/edk/embedder/embedder.h b/mojo/edk/embedder/embedder.h
index b43748ce154e668c0cc5b685172b1bcc1715faaf..2c7f71f89b78422e072509c4924e87ce3c73ecd6 100644
--- a/mojo/edk/embedder/embedder.h
+++ b/mojo/edk/embedder/embedder.h
@@ -118,7 +118,12 @@ MOJO_SYSTEM_IMPL_EXPORT void ShutdownIPCSupportOnIOThread();
 // |OnShutdownComplete()|.
 MOJO_SYSTEM_IMPL_EXPORT void ShutdownIPCSupport();
 
-// Unused. Crashes. Only here for linking.
+// Creates a message pipe over an arbitrary platform channel. The other end of
+// the channel must also be passed to this function. Either endpoint can be in
+// any process.
+//
+// Note that the channel is only used to negotiate pipe connection, not as the
+// transport for messages on the pipe.
 MOJO_SYSTEM_IMPL_EXPORT ScopedMessagePipeHandle
 CreateMessagePipe(ScopedPlatformHandle platform_handle);