Improve wxImage handlers documentation, especially ownership aspect.

Improve wxImage handlers documentation, especially ownership aspect.

Correct wxImage::RemoveHandler() documentation which was plain wrong.

Also mention that the handler will be deleted by wxImage in AddHandler()
documentation.

And add an example of using AddHandler() as well as fix the broken text in
wxInitAllImageHandlers() description.

Closes #12772.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66370 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775