FCPPluginClient.send(): JavaDoc

- Stress that the only way of knowing whether a send succeeded is by
having received a reply.
- Do not make people use sendSynchronous() if they want to know that a
send succeeded. Instead tell them that they can of course also expect
their message handler to receive a reply.
PluginRespirator.connectToOtherPlugin(): JavaDoc grammar fix
PluginRespirator.connectToOtherPlugin(): JavaDoc

- Move comments about disconnection from @return to "Disconnecting
properly" section. We have a whole section dedicated to disconnection,
so it really belongs there.
- Also, clarify the moved JavaDoc to stress very much that a dead
FCPPluginClient cannot be revived.