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

[Congcong Cai via cfe-commits]

https://github.com/HerrCai0907 commented:

Please update `clang-tools-extra/docs/clang-tidy/checks/readability/container-size-empty.rst` also.

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