index-format.txt: document v2 format of file system monitor extension

Update the documentation of the file system monitor extension to
describe version 2.

The format was extended to support opaque tokens in:
56c6910028 fsmonitor: change last update timestamp on the index_state to opaque token

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/index-format.txt

@@ -304,12 +304,18 @@ The remaining data of each directory block is grouped by type:
 
   The extension starts with
 
-  - 32-bit version number: the current supported version is 1.
+  - 32-bit version number: the current supported versions are 1 and 2.
 
-  - 64-bit time: the extension data reflects all changes through the given
+  - (Version 1)
+    64-bit time: the extension data reflects all changes through the given
 	time which is stored as the nanoseconds elapsed since midnight,
 	January 1, 1970.
 
+  - (Version 2)
+    A null terminated string: an opaque token defined by the file system
+    monitor application.  The extension data reflects all changes relative
+    to that token.
+
   - 32-bit bitmap size: the size of the CE_FSMONITOR_VALID bitmap.
 
   - An ewah bitmap, the n-th bit indicates whether the n-th index entry