std::str Adapt documentation to reality

src/libcore/str/mod.rs

@@ -1424,10 +1424,8 @@ fn contains_nonascii(x: usize) -> bool {
     (x & NONASCII_MASK) != 0
 }
 
-/// Walks through `iter` checking that it's a valid UTF-8 sequence,
-/// returning `true` in that case, or, if it is invalid, `false` with
-/// `iter` reset such that it is pointing at the first byte in the
-/// invalid sequence.
+/// Walks through `v` checking that it's a valid UTF-8 sequence,
+/// returning `Ok(())` in that case, or, if it is invalid, `Err(err)`.
 #[inline]
 fn run_utf8_validation(v: &[u8]) -> Result<(), Utf8Error> {
     let mut index = 0;