std: impl of Write for &mut [u8]: document the buffer full error

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

library/std/src/io/impls.rs

@@ -306,6 +306,10 @@ impl BufRead for &[u8] {
 ///
 /// Note that writing updates the slice to point to the yet unwritten part.
 /// The slice will be empty when it has been completely overwritten.
+///
+/// If the number of bytes to be written exceeds the size of the slice, write operations will
+/// return short writes: ultimately, `Ok(0)`; in this situation, `write_all` returns an error of
+/// kind `ErrorKind::WriteZero`.
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Write for &mut [u8] {
     #[inline]