index-format: discuss recursion of cache-tree better

The end of the cache tree index extension format trails off with
ellipses ever since 23fcc98 (doc: technical details about the index
file format, 2011-03-01). While an intuitive reader could gather what
this means, it could be better to use "and so on" instead.

Really, this is only justified because I also wanted to point out that
the number of subtrees in the index format is used to determine when the
recursive depth-first-search stack should be "popped." This should help
to add clarity to the format.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/index-format.txt

@@ -195,7 +195,8 @@ Git index format
   first entry represents the root level of the repository, followed by the
   first subtree--let's call this A--of the root level (with its name
   relative to the root level), followed by the first subtree of A (with
-  its name relative to A), ...
+  its name relative to A), and so on. The specified number of subtrees
+  indicates when the current level of the recursive stack is complete.
 
 === Resolve undo