Change recommendation to use Slf4j's formatting anchors with common ex… (#493)

* Change recommendation to use Slf4j's formatting anchors with common examples and link to FAQ.

* Fix log statment to work in 3.2

Author: erichelgeson

src/en/guide/scaffolding.gdoc

@@ -85,7 +85,7 @@ class BookController {
 
     def show() {
         def book = Book.get(params.id)
-        log.error(book)
+        log.error("{}", book)
         [bookInstance : book]
     }
 }

src/en/guide/upgrading/upgrading31x.gdoc

@@ -16,8 +16,17 @@ Double notAString = 9.2
 log.info notAString
 {code}
 
-The latter call to the `info` method will throw an exception and `toString()` must be called to resolve the issue.
+The latter call to the `info` method will throw an exception as it is not a String.
 
+Instead you should use Slf4j's formatting anchors to log. The advantage is the `toString()` method is not called unless the message will be logged.
+
+{code}
+log.info "{}", 9.2D
+log.debug "Key: {}, Value: {}", key, value
+log.error "{}", exception.message, exception
+{code}
+
+See the [Slf4j FAQ|http://www.slf4j.org/faq.html#string_or_object] for more information.
 
 h4. Spring 4.3