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[MINOR][DOCS] Explicitly mention RDD/SparkContext are unsupported in … #386

Job	Run time
Run / Check changes	41s
Run / Base image build	55s
Run / Protobuf breaking change detection and Python CodeGen check	1m 1s
Run / Build modules: ${{ matrix.modules }} ${{ matrix.comment }}	0s
Run / Run TPC-DS queries with SF=1	0s
Run / Run Docker integration tests	0s
Run / Run Spark UI tests	22s
Run / Run Spark on Kubernetes Integration test	0s
Run / Linters, licenses, and dependencies	27m 12s
Run / Documentation generation	15m 59s
Run / Build modules: sparkr	0s
Run / Build modules: ${{ matrix.modules }}	0s
	46m 10s

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