Document that SVG class manipulation is supported in jQuery 1.12, 2.2 or newer

[mgol]

jQuery doesn't generally support SVG but due to the fact we've received exceptionally many questions about class manipulation support and it was relatively cheap for us to do, we added such support in jQuery 1.12/2.2. We should document that addClass, removeClass, hasClass and toggleClass is now supported on SVG nodes and we have unit tests for that.

The note should also warn that class manip support is an exception and that most jQuery APIs don't support SVGs.

[AurelioDeRosa]

The addClass and removeClass have already a note about SVG support. I'm going to add a note for hasClass and toggleClass.