[algorithms, numerics] Replace "exactly" with "at most" in Complexity

[frederick-vs-ja]

Fixes #5636.

Per the discussion in that issue, I think this can be editorial. Although it seems to me that some "exactly" are actually exact.

[jensmaurer]

@jwakely ?

[jwakely]

This doesn't seem like an improvement. Do we want implementations to be allowed to perform fewer than last - first applications for for_each, as an optimization? Or to perform fewer than last - first assignments for std::copy?

If that's allowed, I can make my implementation much faster.

[jwakely]

I agree that for some cases like ranges::shuffle "exactly" isn't helpful, but I don't see why we'd want to remove "exactly" from for_each, copy, etc.

Is the concern that an exception could cause the loop to finish early? Because I don't think we care about that for the Complexity elements.