Update env vars doc about `XDG_*` variables on macOS #12

Job	Run time
cargo shear	32s
cargo fmt	44s
mkdocs	16s
Determine changes	5s
typos	6s
Python lint	12s
benchmarks	0s
build binary \| macos x86_64	0s
build binary \| linux	0s
build binary \| macos aarch64	0s
cargo clippy \| ubuntu	0s
build binary \| windows	0s
cargo test \| ubuntu	0s
cargo clippy \| windows	0s
cargo test \| macos	0s
cargo test \| windows	0s
check windows trampoline \| ${{ matrix.target-arch }}	0s
test windows trampoline \| ${{ matrix.target-arch }}	0s
check cache \| macos aarch64	0s
ecosystem test \| ${{ matrix.repo }}	0s
check cache \| ubuntu	0s
integration test \| conda on ubuntu	0s
integration test \| github actions	0s
integration test \| graalpy on ubuntu	0s
integration test \| graalpy on windows	0s
integration test \| pypy on ubuntu	0s
integration test \| pypy on windows	0s
check system \| alpine	0s
check system \| amazonlinux	0s
check system \| python3.12 via chocolatey	0s
check system \| conda${{ matrix.python-version }} on ${{ matrix.os }}	0s
check system \| python on debian	0s
check system \| python on fedora	0s
check system \| python on macos aarch64	0s
check system \| homebrew python on macos aarch64	0s
check system \| python on macos x86_64	0s
check system \| python on opensuse	0s
check system \| python via pyenv	0s
check system \| pypy on ubuntu	0s
check system \| pyston	0s
check system \| python on rocky linux ${{ matrix.rocky-version }}	0s
check system \| embedded python3.10 on windows	0s
check system \| python on ubuntu	0s
check system \| python3.10 on windows	0s
check system \| python3.13 on windows	0s
check system \| python3.10 on windows x86	0s
	1m 55s

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