wxDocument::Revert() method existed but didn't do anything, provide a
default implementation for it. Also document it and show it in the sample.
Closes#11849.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63748 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775