[clang-tools-extra] [clang-tidy] Adjust size-empty doc because C++11 size() is constant-time (PR #117629)

[Niels Dekker via cfe-commits]

N-Dekker wrote:

@PiotrZSL @HerrCai0907 Thank you both for your feedback and approval! Would one of you be able the merge the pull request? 

https://github.com/llvm/llvm-project/pull/117629