addClass/removeClass change the property not the attribute

[dmethvin]

Clarify that these two methods manipulate the property and not the attribute. In HTML DOM docs the browser echoes the className property value back into the class attribute. Two implications:

• The browser may not remove the class attribute when the last class name is removed, it may only be set to an empty string.
• The addClass/removeClass methods only work for documents with HTML DOM semantics (e.g., not pure XML documents).