Update documentation wherever a matrix can be provided as callable

[jeverink]

In various locations, e.g., for example CGLS and FISTAin the solvers module, the matrix can be provided as a ndarray and callable and is described in the documentation as

A : ndarray or callable f(x,*args).

It would be helpful to have a more clearer description of what is required from the callable, e.g., like is being done for ADMM as

A : ndarray or callable
Represents a matrix or a function that performs matrix-vector multiplications.
When A is a callable, it accepts arguments (x, flag) where:
- flag=1 indicates multiplication of A with vector x, that is A @ x.
- flag=2 indicates multiplication of the transpose of A with vector x, that is A.T @ x.