Clarify WTF-8 safety docs
This PR is a follow-up to PR #140159, which clarifies ~~two things~~:
- the WTF-8 safety comment [was confusing](https://github.com/rust-lang/rust/pull/140159#discussion_r2082766965), either surrogate condition is actually sufficient for safety, both are not required
- ~~the private `os_str::Slice` type name is easily confused with `std::slice`~~
~~Happy to bikeshed the `OsSlice` name, other alternatives are `OsStrSlice` and `StrSlice`. Now it's got a distinct name from `std::slice`, it's easy to search and replace.~~
cc ``@thaliaarchi`` ``@workingjubilee``