Document that applications should avoid flooding FX with Platform.runLater calls

Numerous api users seem to be abusing the Platform.runLater() api by calling it very often (hundreds of times per second), which causes their applications to behave strangely and they don't know why.

Add a recommended maximum number of calls per minute to the Platform.runLater javadoc so that API users are aware that they should not be invoking this API too often.

For related forum threads and user questions see:
http://stackoverflow.com/questions/10607507/javafx-2-blank-label-refresh-issue
https://forums.oracle.com/forums/thread.jspa?messageID=10170806