Update shellFor documentation (#2406) #3262

Job	Run time
unit-tests	19s
nix-build	33m 15s
maintainer-scripts	17s
cabal-doExactConfig	5m 7s
tests-benchmarks	1m 25s
multi-target	13m 28s
runghc	7m 13s
plan-extra-hackages	19s
shellFor-single-package	6m 21s
shellFor-multiple-package	9m 22s
cabal	5m 53s
shellFor-not-depends	46s
shellFor-hoogle	5m 3s
hydra-without-remote-builders-ghc9102	8m 47s
build-extra-hackages	16s
hydra-ifdLevel-3	30m 17s
hydra-ifdLevel-0-and-1	11m 10s
closure-size	8s
check-nix-tools	31s
hix-cabal	7m 52s
docs	32m 24s
hix	11m 58s
hydra-ifdLevel-2	18m 51s
update-docs	13s
check-materialization-concurrency	26s
hydra-without-remote-builders-ghc967	7m 21s
haskell-nix-roots-do-not-require-IFDs	32s
check-path-support	24s
	3h 39m 58s

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