The entire submodule section in the superproject will be per-worktree
and written to $GIT_DIR/config.worktree.

The behavior when you only have one worktree (and not enabled
extensions.worktreeConfig) is the same as before, everything is still
written in $GIT_DIR/config

If you have multiple worktrees without extensions.worktreeConfig(),
you'll face this message:

fatal: submodules cannot be used with multiple working trees unless the
config extension worktreeConfig is enabled. Please read "CONFIGURATION
FILE" section in "git help worktree" for details