Fix up docs for String::from_utf8_lossy()

Fix up docs for String::from_utf8_lossy()

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/