clarify some FS comments

file.h

@@ -9,8 +9,7 @@ struct file {
 };
 
 
-// in-core file system types
-
+// in-memory copy of an inode
 struct inode {
   uint dev;           // Device number
   uint inum;          // Inode number
@@ -24,12 +23,11 @@ struct inode {
   uint size;
   uint addrs[NDIRECT+1];
 };
-
 #define I_BUSY 0x1
 #define I_VALID 0x2
 
-// device implementations
-
+// table mapping major device number to
+// device functions
 struct devsw {
   int (*read)(struct inode*, char*, int);
   int (*write)(struct inode*, char*, int);