[3.1.0] More PHP4->PHP5 conversions, notably reference removal of most methods that return objects

- Removed HTMLPurifier_Error
- Documentation updates
- Removed more copy() methods in favor of clone
- HTMLPurifier::getInstance() to HTMLPurifier::instance()
- Fix InterchangeBuilder to use HTMLPURIFIER_PREFIX

git-svn-id: http://htmlpurifier.org/svnroot/htmlpurifier/trunk@1689 48356398-32a2-884e-a903-53898d9a118a

library/HTMLPurifier/AttrDef.php

@@ -70,9 +70,10 @@ abstract class HTMLPurifier_AttrDef
      * @return Created AttrDef object corresponding to $string
      */
     public function make($string) {
-        // default implementation, return flyweight of this object
-        // if overloaded, it is *necessary* for you to clone the
-        // object (usually by instantiating a new copy) and return that
+        // default implementation, return a flyweight of this object.
+        // If $string has an effect on the returned object (i.e. you
+        // need to overload this method), it is best
+        // to clone or instantiate new copies. (Instantiation is safer.)
         return $this;
     }