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

docs/changes.txt

@@ -80,6 +80,11 @@ Changes in behaviour not resulting in compilation errors, please read this!
 - Default location of wxFileConfig files has changed under Windows, you will
   need to update your code if you access these files directly.
 
+- Return value of wxStandardPaths::GetUserDataDir() and similar functions has
+  changed if the vendor name is defined as it now includes it by default under
+  Windows and OS X. Call wxStandardPaths::UseAppInfo(AppInfo_AppName) to
+  revert to behaviour compatible with 2.8 version.
+
 - wxWindow::IsEnabled() now returns false if a window parent (and not
   necessarily the window itself) is disabled, new function IsThisEnabled()
   with the same behaviour as old IsEnabled() was added.