stylix: simplify `mkEnableTarget` documentation #710

Job	Run time
List packages	25s
docs on x86_64-darwin	1m 15s
palette-generator on x86_64-darwin	2m 28s
docs on x86_64-linux	46s
palette-generator on x86_64-linux	25s
testbed-gnome-dark on x86_64-linux	3m 56s
testbed-gnome-light on x86_64-linux	4m 23s
testbed-kde-dark on x86_64-linux	2m 26s
testbed-kde-light on x86_64-linux	2m 23s
	18m 27s

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