erofs: update erofs_fs.h comments

As Christoph said [1] [2], update it now.

[1] https://lore.kernel.org/r/20190902124521.GA22153@infradead.org/
[2] https://lore.kernel.org/r/20190902120548.GB15931@infradead.org/Reported-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Link: https://lore.kernel.org/r/20190904020912.63925-11-gaoxiang25@huawei.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/erofs/erofs_fs.h

 /* SPDX-License-Identifier: GPL-2.0-only OR Apache-2.0 */
 /*
+ * EROFS (Enhanced ROM File System) on-disk format definition
+ *
  * Copyright (C) 2017-2018 HUAWEI, Inc.
  *             http://www.huawei.com/
  * Created by Gao Xiang <gaoxiang25@huawei.com>
@@ -7,7 +9,6 @@
 #ifndef __EROFS_FS_H
 #define __EROFS_FS_H
 
-/* Enhanced(Extended) ROM File System */
 #define EROFS_SUPER_OFFSET      1024
 
 /*
@@ -41,7 +42,7 @@ struct erofs_super_block {
 };
 
 /*
- * erofs inode datalayout:
+ * erofs inode datalayout (i_format in on-disk inode):
  * 0 - inode plain without inline data A:
  * inode, [xattrs], ... | ... | no-holed data
  * 1 - inode VLE compression B (legacy):
@@ -187,7 +188,7 @@ static inline unsigned int erofs_xattr_entry_size(struct erofs_xattr_entry *e)
 				 e->e_name_len + le16_to_cpu(e->e_value_size));
 }
 
-/* available compression algorithm types */
+/* available compression algorithm types (for h_algorithmtype) */
 enum {
 	Z_EROFS_COMPRESSION_LZ4	= 0,
 	Z_EROFS_COMPRESSION_MAX
@@ -222,7 +223,7 @@ struct z_erofs_map_header {
 #define Z_EROFS_VLE_LEGACY_HEADER_PADDING       8
 
 /*
- * Z_EROFS Variable-sized Logical Extent cluster type:
+ * Fixed-sized output compression ondisk Logical Extent cluster type:
  *    0 - literal (uncompressed) cluster
  *    1 - compressed cluster (for the head logical cluster)
  *    2 - compressed cluster (for the other logical clusters)