PluginRespirator.connectToOtherPlugin(): JavaDoc

Stress that FCPPluginClients may be re-used to send multiple messages.
This is important to be stressed because during code review of WOT
client plugins I have encountered developers creating a fresh
PluginTalker (= what was replaced by FCPPluginClient) for every message.
PluginRespirator.connectToOtherPlugin(): JavaDoc

Remove recommendation of pinging by FCPPluginClient.sendSynchronous().
sendSynchronous() is a rather heavy function as it keeps the thread
alive.
Pinging can be done without a live thread by having a timer which wakes
up the keepalive loop, and storing the "last time a ping was received"
date in a member variable. No need for a thread there.
Also, I guess explaining how to implement pinging is beyond the scope of
the JavaDoc of this function anyway - its basic networking knowhow.
PluginRespirator.connectToOtherPlugin(): JavaDoc

We don't know anything about server plugin ping messages: They might
have a different name, but still serve the sole purpose of being a ping.
So "implicit" is speculative here, and is removed by this commit.