docs: BytesMut::with_capacity does not guarantee exact capacity (#5870)

Author: mahkoh

tokio/src/io/util/async_read_ext.rs

@@ -230,10 +230,13 @@ cfg_io_util! {
         ///     let mut buffer = BytesMut::with_capacity(10);
         ///
         ///     assert!(buffer.is_empty());
+        ///     assert!(buffer.capacity() >= 10);
         ///
-        ///     // read up to 10 bytes, note that the return value is not needed
-        ///     // to access the data that was read as `buffer`'s internal
-        ///     // cursor is updated.
+        ///     // note that the return value is not needed to access the data
+        ///     // that was read as `buffer`'s internal cursor is updated.
+        ///     //
+        ///     // this might read more than 10 bytes if the capacity of `buffer`
+        ///     // is larger than 10.
         ///     f.read_buf(&mut buffer).await?;
         ///
         ///     println!("The bytes: {:?}", &buffer[..]);