FCPPluginClient.send()/sendSynchronous(): JavaDoc parameter direction

- The send() version was inspired by the version of sendSynchronous()
- Some improvements came to my mind which the commit also applies to the
sendSynchronous() version:
* Text with <b> is more clearly visible than <i>, so I think we should
only use <b>.
* The fact that the thread is number two is not important, its only
important that it is a different thread
* Deadlocks can not only happen explicitely with Lock objects, but also
implicitely by having threads which wait for certain events in a circle.
So its  rather "measures" than "objects" what can cause deadlocks.