Fix minor typo in wxDV_VERT_RULES documentation.

Don't say "is supported" when meaning "if supported" and, in fact, remove this
part entirely as these styles are supported by all implementations.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73676 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775