[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:

A personal note: yes, I like clang-tidy's readibility-container-size-empty check, I think it's great! And I do very much prefer `empty()` over `size()` "whenever possible". It's just that I think the claims in the documentation about efficiency and time complexity are no longer valid.

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