Add reminder to get rid of `arc_make_mut_slice()`. #619

Job	Run time
incremental-mutants	0s
miri	1m 37s
build (ubuntu, stable, locked, true)	14m 44s
deny	1m 8s
build (windows, stable, locked, false)	26m 14s
build (macos, stable, locked, false)	10m 43s
no_std	41s
build (ubuntu, nightly, locked, false)	22m 4s
build (ubuntu, beta, locked, false)	22m 29s
build (ubuntu, stable, latest, false)	16m 35s
build (ubuntu, stable, minimal, false)	17m 43s
deploy	8s
features	29m 53s
fuzz	15m 23s
	2h 59m 22s

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