Document optimization fuel for MIR optimizations

[cjkenn]

This comment suggests adding some docs for optimization fuel usage. I agree with that sentiment, so now that rust-lang/rust#79117 is merged I figure we can whip up a quick explanation for the change and why optimization fuels is used.

I'll write these docs, just wanted to make this issue in case it's needed for tracking purposes.

[camelid]

@rustbot assign @cjkenn

[jyn514]

@cjkenn thanks for working on this! In general there's no need to open an issue if you're going to immediately open a PR, I normally use issues for things I don't have time for at the moment.