FCPInterface.handleSubscribe(): Fix half-finished JavaDoc linebreaks
FCPInterface.handleSubscribe(): Fix various JavaDoc glitches

- The quoted stuff is entries of FCPPluginMessage.params. Mention that.
- More linebreak fixes.
- Use HTML h1/h2 for clarifying what is about event notifications
- Do not talk about the "synchronous" FCP API: The mechanism of replying
to an FCP message with a success==true/false field is independent of the
"synchronous" API. And "synchronization" is already used for a
completely different concept here (BeginSynchronizationEvent etc.), so
we better not mix that up with the synchronous FCP stuff.
SubscriptionManager: Add performance TODO

InterruptedException is merely declared, not thrown.