Minor: Add additional documentation to CommonSubexprEliminate

[alamb]

Which issue does this PR close?

Related to #9637 and #3058

Rationale for this change

As part of #9637 I plan to rework CommonSubexprEliminate a bit to stop copying as much

I started looking at this code, and I wanted to document how it worked more clearly (mostly for my future self, but I think it will be more generally useful)

What changes are included in this PR?

1. Add doc comments in CommonSubexprEliminate

Are these changes tested?

Doc CI tests

Are there any user-facing changes?

Only some minor docstring changes. No functional changes intended

[alamb]

FYI @wiedld and @waynexia I think you have worked on this code at some point so may be interested in this PR

[comphead]

Nice thanks @alamb I was thinking if we need to have an optimizer rules doc with just list of rules and description what every rule is supposed to do

[alamb]

Nice thanks @alamb I was thinking if we need to have an optimizer rules doc with just list of rules and description what every rule is supposed to do

I agree -- this would help. I'll spend a few minutes updating our docs with that info

Update: #9967