index-format: discuss recursion of cache-tree better

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>