QRegularExpression: add docs note about how "-" in [] is treated differently than with QRegExp

With QRegExp patterns like [\w-9] were valid, IIUC, there weren't treated as a range of characters and QRegExp didn't complain about it.

But with QRegularExpression such a pattern is invalid; so with QRegularExpression:

• To match a literal "-" in [] it has to be right after [, escaped or right before ]
• To match a range of characters, it has be used correctly, and QRegularExpression isn't as forgiving as QRegExp is

I suggest adding a porting note to the docs about this issue. (And sorry if I got anything wrong up there ).

Thanks in advance.