DOCS: solver callback function signatures insufficiently documented

[connorjward]

Variational solvers accept pre_ and post_ callback functions. The docstring for NonlinearVariationalSolver describes what the pre_ callbacks should look like:

        To use the ``pre_jacobian_callback`` or ``pre_function_callback``
        functionality, the user-defined function must accept the current
        solution as a petsc4py Vec. Example usage is given below:

        .. code-block:: python3

            def update_diffusivity(current_solution):
                with cursol.dat.vec_wo as v:
                    current_solution.copy(v)
                solve(trial*test*dx == dot(grad(cursol), grad(test))*dx, diffusivity)

            solver = NonlinearVariationalSolver(problem,
                                                pre_jacobian_callback=update_diffusivity)

But the post_ callbacks are not mentioned. This is confusing because the function signature for post_ callbacks is different to the pre_ ones. The post_jacobian_callback signature for instance looks like:

def callback(current_solution, J):
    ...

Using the wrong function signature causes the solver to terminate without a helpful error message.

It might be helpful to look at the test_custom_callbacks regression test to see intended usage.