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

[Piotr Zegar via cfe-commits]

PiotrZSL wrote:

You understand that this check does not apply only to std:: containers but also to boost and other custom one that have size and empty methods. In such case claim is still valid. If you change check description, then check documentation also should be updated to be in sync.

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