Updated VM-bridges document

[Brian Goetz]

> 
> The other thing is that Forwarding bridge should not use an adapter but a bootstrap method.

Can you explain exactly what you mean here?  Because in my mind, the adapter _is_ a bootstrap method — it is code to which the VM upcalls at preparation / link time to help establish linkage.  

Since you obviously have a more specific notion of “bootstrap method”, can you explain exactly what you mean?