[mismatch] LWG 3178: std::mismatch is missing an upper bound

[mclow]

https://cplusplus.github.io/LWG/issue3178
This was opened by @geoffromer , but @Dani-Hub thought it might be editorial.

[CaseyCarter]

FWIW, as the last person to touch [mismatch] I can verify that the PR clarifies the intent of the existing wording.

[Dani-Hub]

I will make a note to LWG 3178 regarding these editorial changes.