Use SLF4J in logging guide

[mrts]

The following idiom is recommended in the logging guide:

if (log.isEnabled(Logger.Level.TRACE)) {
    log.trace("called method with: " + name);
}

SLF4J supports a feature called parameterized logging which achieves the same result in a more elegant and succinct way on one line:

log.trace("called method with: {}", name);

As SLF4J binding to JBoss Logging is provided by Wildfly by default, I suggest that we recommend to use SLF4J in the logging guide instead to advocate clean code. Here is the corresponding adapter that proves that the two code snippets above are equivalent.

[hpehl]

Thanks for the hint @mrts. I've opened #541 to get this merged.