copy_to_bytes: Add panic section to docs

Fixes tokio-rs#454.

src/buf/buf_impl.rs

@@ -1120,6 +1120,10 @@ pub trait Buf {
     /// let bytes = (&b"hello world"[..]).copy_to_bytes(5);
     /// assert_eq!(&bytes[..], &b"hello"[..]);
     /// ```
+    ///
+    /// # Panics
+    ///
+    /// This function panics if `len > self.remaining()`.
     fn copy_to_bytes(&mut self, len: usize) -> crate::Bytes {
         use super::BufMut;