[documentation] Change the category of USYMLQ and USYMQR

[amontoison]

Currently USYMLQ and USYMQR are in the category Non-Hermitian indefinite linear systems but we could move USYMLQ into the category Least-norm problems and USYMQR into Least-squares problems.

[dpo]

Yes, that would make more sense. It should also be explained somewhere that both could be used to solve square linear systems (consistent in the case of USYMLQ).