Add comment with `_memoryType`'s purpose #812

Job	Run time
checks / check-style	1m 0s
checks / other-checks	37s
wheel-build-distributed-ucxx / compute-matrix	1s
conda-cpp-build / compute-matrix	1s
wheel-build-ucxx / compute-matrix	1s
conda-cpp-build / ${{ matrix.CUDA_VER }}, ${{ matrix.PY_VER }}, ${{ matrix.ARCH }}, ${{ matrix.LINUX_VER }}	1s
wheel-build-ucxx / ${{ matrix.CUDA_VER }}, ${{ matrix.PY_VER }}, ${{ matrix.ARCH }}, ${{ matrix.LINUX_VER }}	1s
wheel-build-distributed-ucxx / ${{ matrix.CUDA_VER }}, ${{ matrix.PY_VER }}, ${{ matrix.ARCH }}, ${{ matrix.LINUX_VER }}	1s
conda-python-tests	1s
docs-build	1s
conda-cpp-tests	1s
conda-python-distributed-tests	1s
wheel-tests-ucxx	1s
wheel-tests-distributed-ucxx	1s
pr-builder	1s
	1m 50s

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