added some documentation to the function header

lib/layouts.js

@@ -140,9 +140,22 @@ function messagePassThroughLayout (loggingEvent) {
  *  - %d date in various formats
  *  - %% %
  *  - %n newline
+ *  - %x{<tokenname>} add dynamic tokens to your log. Tokens are specified in the tokens parameter
  * You can use %[ and %] to define a colored block.
- * Takes a pattern string and returns a layout function.
+ *
+ * Tokens are specified as simple key:value objects. 
+ * The key represents the token name whereas the value can be a string or function
+ * which is called to extract the value to put in the log message. If token is not
+ * found, it doesn't replace the field.
+ *
+ * A sample token would be: { "pid" : function() { return process.pid; } }
+ *
+ * Takes a pattern string, array of tokens and returns a layout function.
+ * @param {String} Log format pattern String
+ * @param {object} map object of different tokens
+ * @return {Function}
  * @author Stephan Strittmatter
+ * @author Jan Schmidle
  */
 function patternLayout (pattern, tokens) {
     var TTCC_CONVERSION_PATTERN  = "%r %p %c - %m%n";