Document wxStandardPaths vendor-related changes.

Document wxStandardPaths vendor-related changes.

The change in the values returned by wxStandardPaths functions is important as
it's incompatible with 2.8 if the application defines a vendor name so must be
mentioned in docs/changes.txt.

Also update the documentation of the class itself and mention UseAppInfo() in
the overview.

Closes #11189.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61883 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775