Add comments in ch6

easy-fs/src/block_dev.rs

@@ -1,6 +1,9 @@
 use core::any::Any;
-
+/// Trait for block devices
+/// which reads and writes data in the unit of blocks
 pub trait BlockDevice: Send + Sync + Any {
+    ///Read data form block to buffer
     fn read_block(&self, block_id: usize, buf: &mut [u8]);
+    ///Write data from buffer to block
     fn write_block(&self, block_id: usize, buf: &[u8]);
 }