`constrained-generators`: Improve haddocks #19004

Job	Run time
shellcheck	8s
build (9.6.7, ubuntu-latest)	3m 47s
fourmolu	45s
cabal-format	6s
build (9.12.2, ubuntu-latest)	3m 57s
build (9.10.2, ubuntu-latest)	3m 56s
gen-hie	3m 56s
build (9.8.4, ubuntu-latest)	3m 55s
Check branch history	12s
Check if formal-ledger-specifications SRP is valid	13s
codegen	1s
${{ matrix.package }}-${{ matrix.ghc }}-${{ matrix.os }}	1s
Tests completed	4s
Send a slack notification on \#ledger-internal if the nightly build failed	1s
	21m 2s

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