PRDOC: Document `validate: false` #17800

Job	Run time
preflight / preflight	1m 21s
preflight / ci-versions	1m 3s
check-runtime-migration (westend)	6m 17s
check-runtime-migration (asset-hub-westend)	4m 40s
check-runtime-migration (bridge-hub-westend)	4m 5s
check-runtime-migration (collectives-westend)	4m 2s
check-runtime-migration (coretime-westend)	4m 11s
All runtime migrations passed	1s
	25m 40s

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