Fix #213 Add threadsafe note for pretty printer

Author: EdgeCaseBerg

shared/src/main/scala/scala/xml/PrettyPrinter.scala

@@ -21,6 +21,7 @@ import Utility.sbToString
  *  @author  Burak Emir
  *  @param width the width to fit the output into
  *  @param step  indentation
+ *  @note This class is not threadsafe and should not be accessed by multiple threads at the same time.
  */
 class PrettyPrinter(width: Int, step: Int, minimizeEmpty: Boolean) {