MINOR: Update null values documentation in `csv.rst` #2780

Job	Run time
check-labels / Check labels	27s
JNI macOS	0s
JNI ${{ matrix.platform.runs-on }} ${{ matrix.platform.arch }}	0s
ARM64 Windows 11 MSVC	0s
matrix.title	0s
report-extra-cpp	0s
	27s

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