`affineboundary` examples in package docs

[chiyahn]

Creating a separate issue for affineboundary examples in package docs from #188

• [x] This example is a little confusing since it is not self-contianed. i.e. the pi and l, : https://quantecon.github.io/SimpleDifferentialOperators.jl/dev/examples/#Solving-HJBE-with-absorbing-barrier-conditions-1
• [x] Modify the example in https://quantecon.github.io/SimpleDifferentialOperators.jl/dev/examples/#Solving-HJBE-with-absorbing-barrier-conditions-1 to support S > 0 for the 2nd part of the example.
• [x] Also, the text that begins with We can then apply one Gaussian elimination step... is a little weird there because it doesn't look like you use the resulting pi*. I don't think that algebra should be there, but rather in the derivation (which this section can refer to).
• [ ] Document and/or doublecheck the stacked affine fudges in the same order as the joint operator