Minor correction/clarification to LSCG solver documentation

This applies some minor corrections/clarifications to the docs of LeastSquaresConjugateGradient (LSCG).

LSCG solves the least squares problem "min |Ax-b|" or, equivalently, the normal equations "A'Ax=A'b". The documentation seems a little confused about this. In one place, it is described as solving "min |A'Ax-b|", which appears to be a mashup of the two formulas and is not correct. In another place, it is described as solving Ax=b, which is misleading without mentioning least-squares, as it (probably) will not be able to find an x satisfying that equation. I would guess both of these issues are simple copy-paste/editing errors.