Documentation: fix default value for core.maxTreeDepth

Documentation: fix default value for core.maxTreeDepth

When `core.maxTreeDepth` was originally introduced via be20128bfa (add
core.maxTreeDepth config, 2023-08-31), its default value was 4096.

There have since been a couple of updates to its default value that were
not reflected in the documentation for `core.maxTreeDepth`:

  - 4d5693ba05 (lower core.maxTreeDepth default to 2048, 2023-08-31)
  - b64d78ad02 (max_tree_depth: lower it for MSVC to avoid stack
    overflows, 2023-11-01)

Commit 4d5693ba05 lowers the default to 2048 for platforms with smaller
stack sizes, and commit b64d78ad02 lowers the default even further when
Git is compiled with MSVC.

Neither of these changes were reflected in the documentation, which I
noticed while merging newer releases back into GitHub's private fork
(which contained the original implementation of `core.maxTreeDepth`).

Update the documentation to reflect what the platform-specific default
values are.

Noticed-by: Keith W. Campbell <keithc@ca.ibm.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>