When I last did a pass through the string documentation, I focused on consistency across similar functions. Unfortunately, I missed some details. This example was _too_ consistent: it wasn't actually accurate! This commit fixes the docs do both be more accurate and to explain why the return type is a Cow<'a, str>. First reported here: https://www.reddit.com/r/rust/comments/44q9ms/stringfrom_utf8_lossy_doesnt_return_a_string/