Qt Creator doesn't come with STL documentation because Qt has some "replacement" for the STL's containers, iterators and algorithm.
Like you can use (and it's recommended to use) QString instead std::string or std::wstring.

std is the name of the namespace which contains the standard library so that if you have your own class named <for example> string, you don't need to change your class name, because it won't conflict with the standard library std::string (unless you are using namespace std; //thats way it is not recommended to use the std namespace )

I think that knowing STL will help you better understand Qt (especially containers-iterator-algorithm), but it is not necessary to know STL for Qt development;
if you learned STL in Qt you can do one confusion which i did a lot: QList is not a linkedlist like std::list, qt has QLinkedList