doc: don't mention `Drain` specifically in the feature docs #85

Job	Run time
test	2m 24s
clippy-benches	28s
format	8s
clippy (1.64.0, --no-default-features)	1m 30s
clippy (stable, --no-default-features)	9s
clippy (nightly, --all-features, --tests)	33s
	5m 12s

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