man/checkers/variableScope.md: rewrite description and motivation (#8… #20946

Job	Run time
build_make (alpine:3.23)	6m 44s
build_make (ubuntu:24.04)	4m 50s
build_make (ubuntu:25.10)	5m 41s
build_cmake (ubuntu:24.04, true, true)	4m 36s
build_cmake (ubuntu:25.10, true, true)	6m 25s
build_cmake (alpine:3.23, false, false)	3m 3s
	31m 19s

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