Update counted_iterator's "same sequence" definition

[CaseyCarter]

Per LWG-3543. (This is almost a pure non-functional change, but I added a word to the error message we emit on failure.)

Fixes #2184

[StephanTLavavej]

@CaseyCarter I pushed a change to add the "(possibly past-the-end)" that appears in the Standard, as when I see a Standard paragraph citation followed by quotes, I expect the quote to be exact (unless it contains "[...]" or something says that it's paraphrased).

I extracted the period purely for style/consistency, although the original way wasn't wrong (as the period really does exist in the Standardese). The second quote is more clearly a "paraphrasing the Standard" usage so I'm not concerned that it doesn't exactly match the Standardese.

If including the parenthetical part is a distraction, please meow!

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

Thanks for taking care of this LWG issue resolution! 🚀 😸 ✅