Clarify that optimization level 1

[davidspies]

It also turns on debug_assertions.
(I was confused by this)

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[ehuss]

Thanks for the PR! I don't think this is quite correct, though I see how it is confusing.

In Cargo, debug-assertions are always driven by the profile (it doesn't have any default behavior based on the opt-level), and the profile has hard-coded defaults for things like debug-assertions. I think when I wrote this, I probably forgot that and just copied the behavior as that is how rustc itself works.

I think the correct fix here is to just remove the note also turns on….

[joshtriplett]

I just filed a PR to update the language as you suggested, @ehuss.