Documentation: clarify the invalidated tree entry format

When the entry_count is -1, the tree is invalidated and therefore has
not associated hash (or object name). Explicitly state that the next
entry starts after the newline.

Signed-off-by: Carlos Martín Nieto <cmn@elego.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/index-format.txt

@@ -147,8 +147,9 @@ GIT index format
   - 160-bit object name for the object that would result from writing
     this span of index as a tree.
 
-  An entry can be in an invalidated state and is represented by having -1
-  in the entry_count field.
+  An entry can be in an invalidated state and is represented by having
+  -1 in the entry_count field. In this case, there is no object name
+  and the next entry starts immediately after the newline.
 
   The entries are written out in the top-down, depth-first order.  The
   first entry represents the root level of the repository, followed by the