Documentation: Square matrices

Several of the reported algorithms (`bicgstabl`, `gmres`) explicitly say that `A` be a "general, linear operator"; others (`idrs`) at least talk about a "nonsymmetric linear system". The only place where I can find something mentioned about the dimensions of `A` is the home of the documentation, where there is a single, easily overlooked sentence: "When solving linear systems `A x = b` for a square matrix `A` there are quite some options." The rest of the documentation doesn't ever mention the necessity of square matrices again. I would propose to add this very explictly to all the algorithms that speak about so much generality - and maybe on the Home, also emphasize the "square" part, given that the table below has the structure requirements all highlighted in bold.