doc: console example improvement

Reviewed-by: Trevor Norris <trev.norris@gmail.com>
Reviewed-by: Fedor Indutny <fedor@indutny.com>

doc/api/console.markdown

@@ -27,7 +27,9 @@ should worry about unless you log huge amounts of data.
 Prints to stdout with newline. This function can take multiple arguments in a
 `printf()`-like way. Example:
 
+    var count = 5;
     console.log('count: %d', count);
+    // prints 'count: 5'
 
 If formatting elements are not found in the first string then `util.inspect`
 is used on each argument.  See [util.format()][] for more information.
@@ -73,6 +75,7 @@ Finish timer, record output. Example:
       ;
     }
     console.timeEnd('100-elements');
+    // prints 100-elements: 262ms
 
 ## console.trace(message, [...])