index-format: update preamble to cache tree extension

index-format: update preamble to cache tree extension

I had difficulty in my efforts to learn about the cache tree extension
based on the documentation and code because I had an incorrect
assumption about how it behaved. This might be due to some ambiguity in
the documentation, so this change modifies the beginning of the cache
tree format by expanding the description of the feature.

My hope is that this documentation clarifies a few things:

1. There is an in-memory recursive tree structure that is constructed
   from the extension data. This structure has a few differences, such
   as where the name is stored.

2. What does it mean for an entry to be invalid?

3. When exactly are "new" trees created?

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