`constrained-generators`: Improve haddocks #18950

Job	Run time
cabal-format	8s
fourmolu	45s
shellcheck	5s
Check branch history	11s
gen-hie	59s
build (9.8.4, ubuntu-latest)	49s
build (9.12.2, ubuntu-latest)	1m 0s
build (9.10.2, ubuntu-latest)	1m 1s
build (9.6.7, ubuntu-latest)	59s
Check if formal-ledger-specifications SRP is valid	16s
Tests completed	2s
codegen	1s
Send a slack notification on \#ledger-internal if the nightly build failed	1s
${{ matrix.package }}-${{ matrix.ghc }}-${{ matrix.os }}	1s
	6m 18s

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