Futher streamline qHash() documentation

Following the addition of the \qhash family of qdoc macros, there remain inconsistencies even with the macro being used:

• Traditionally, most qHash() overloads have been \relates QHash, but this doesn't work with qHash() implemented as hidden friends (which is what we decided to go with in Qt 6, but haven't, yet, fully implemented), because hidden friends are always related to their enclosing class. Anyway, it would be nice if both the type's class documentation would say whether the type is hashable, and the QHash docs would have a list of (Qt) types that are hashable. So we should either have an \relatesalso, or we need to put hashable types into a group and list that group in QHash docs.