Expand docstrings of linear solvers #470

We should add the following information to the docstrings in psydac/linalg/solvers.py:

• Clarify whether left/right/two-sided preconditioner is used
• Give reference to original algorithm (book/paper/Wikipedia?), write it down in LaTeX, and show it in documentation
• Clarify exit conditions (which residual, normalization, etc...)