FInish off the text for the time-varying bellman example

[jlperla]

• Add in a description of how the spatial discretization turns into a system of ODE, reorganizing the equations/etc.
• Ensure the "backwards in time" description is clear. Look at the PTW computational appendix without the algebraic equations.

[jlperla]

We can hold off on this for now and get ready to push the docs.