QDir::cleanPath(): Mention that the separator conversion only occurs on platforms that use other than / as a path separator

QDir::cleanPath() documentation is a bit misleading:

https://doc.qt.io/qt-5/qdir.html#cleanPath

 
Returns path with directory separators normalized (converted to "/") 

This could be interpreted so that path separators are always converted to /, but this only happens on platforms that don't have / as a path separator.