Improve logging documentation w.r.t. the performance advices from Laurent Bourges:

Bad practices:
- string concat:
    log.fine("message" + value); // means StringBuilder(message).append(String.valueOf(value)).toString(): 2 objects created and value.toString() called
- varags:
    log.fine("message {0}", this); // create an Object[]

Best practices:
if (log.isLoggable(PlatformLogger.FINE) {
    log.fine("message" + value);
}

if (log.isLoggable(PlatformLogger.FINE) {
    log.fine("message {0}", this);
}