Add values of default arguments in docstrings

[frapac]

That allows to read the default values of the arguments with the help?> function, directly in Julia's REPL.

Note: By default, maxit is set to 2 * length(b) whenever maxit=0. Should we specify this value directly in the args?

[amontoison]

If the matrix is rectangular itmax=n+m with (n,m) = size(A). But we should add in the doc that itmax=0means itmax=2n if the matrix is square, otherwise itmax=n+m.

[frapac]

I see. I will update the PR accordingly.

[dpo]

Thank you!