`constrained-generators`: Improve haddocks #18920

Job	Run time
fourmolu	43s
build (9.6.7, ubuntu-latest)	6m 4s
shellcheck	5s
cabal-format	6s
build (9.12.2, ubuntu-latest)	4m 47s
build (9.10.2, ubuntu-latest)	4m 32s
Check branch history	13s
build (9.8.4, ubuntu-latest)	6m 4s
Check if formal-ledger-specifications SRP is valid	12s
gen-hie	7m 9s
codegen	0s
Tests completed	3s
${{ matrix.package }}-${{ matrix.ghc }}-${{ matrix.os }}	0s
Send a slack notification on \#ledger-internal if the nightly build failed	0s
	29m 58s

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