Improve solver documentation

[patrick-kidger]

• Add the nonlinear_solver argument to the documentation for the implicit methods.
• Add the scan_stages argument to the documention for RK methods.
• Link the midpoint method referred to in the LeapfrogMidpoint documentation to our midpoint method.
• Link thr third-order Heun method referred to in the Heun documentation to Bosh3
• State in thr Bosh3 documentation that it's Heun's third order method.