Update documentation for fill_buf in std::io::BufRead

Brings the documentation in line with the BufReader implementation.

Fixes rust-lang#48022.

src/libstd/io/mod.rs

@@ -1330,7 +1330,8 @@ fn read_until<R: BufRead + ?Sized>(r: &mut R, delim: u8, buf: &mut Vec<u8>)
 ///
 #[stable(feature = "rust1", since = "1.0.0")]
 pub trait BufRead: Read {
-    /// Fills the internal buffer of this object, returning the buffer contents.
+    /// Returns the contents of the internal buffer, filling it with more data
+    /// from the inner reader if it is empty.
     ///
     /// This function is a lower-level call. It needs to be paired with the
     /// [`consume`] method to function properly. When calling this