docs: document zero bits in index "mode"

Documentation/gitformat-index.txt describes the "mode" as 32 bits, but
only documents 16 bits. Document the missing 16 bits and specify that
'unused' bits must be zero.

Signed-off-by: Glen Choo <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: chooglen

Documentation/gitformat-index.txt

@@ -83,11 +83,13 @@ Git index format
 
   32-bit mode, split into (high to low bits)
 
+    16-bit unused, must be zero
+
     4-bit object type
       valid values in binary are 1000 (regular file), 1010 (symbolic link)
       and 1110 (gitlink)
 
-    3-bit unused
+    3-bit unused, must be zero
 
     9-bit unix permission. Only 0755 and 0644 are valid for regular files.
     Symbolic links and gitlinks have value 0 in this field.