Update doc of qrfact

At long last, the documentation for the QR factorization (qrfact) is now in line with reality.
- Explains which of QR, QRCompactWY and QRPivoted is returned depending on the inputs
- Explains what QRPackedQ and QRCompactWYQ are and what you can use them for
- Some technical notes on the structure of all these objects and why we have so many of them