Doc: Add documentation for `AsyncRuntime::Watch` traits #4026

Job	Run time
openraft-feature-test (nightly)	50s
examples (stable, memstore)	40s
cluster-benchmark (cluster_benchmark)	48s
openraft-feature-test (nightly, serde)	52s
lint	12m 6s
examples (stable, raft-kv-memstore)	2m 29s
openraft-feature-test (nightly, single-term-leader)	57s
examples (stable, raft-kv-memstore-network-v2)	1m 0s
openraft-feature-test (nightly, single-term-leader,serde,singlethreaded)	51s
examples (stable, raft-kv-memstore-opendal-snapshot-data)	1m 3s
openraft-test (stable, 0)	4m 5s
openraft-test-bench (nightly)	1m 0s
examples (stable, raft-kv-memstore-singlethreaded)	51s
openraft-test (nightly, 30)	3m 32s
rt-monoio	46s
examples (stable, raft-kv-rocksdb)	11m 40s
openraft-test (nightly, 0, single-term-leader)	3m 14s
examples (nightly, memstore)	58s
stores (stores/memstore)	54s
examples (nightly, raft-kv-memstore)	2m 12s
stores (stores/rocksstore)	6m 27s
Build (nightly, bench,serde,bt,singlethreaded)	49s
examples (nightly, raft-kv-memstore-network-v2)	1m 5s
tests-feature-test (nightly)	2m 58s
stores (stores/sledstore)	1m 3s
tests-feature-test (nightly, single-term-leader)	3m 3s
examples (nightly, raft-kv-memstore-opendal-snapshot-data)	1m 30s
examples (nightly, raft-kv-memstore-singlethreaded)	1m 7s
tests-feature-test (nightly, loosen-follower-log-revert)	3m 0s
examples (nightly, raft-kv-rocksdb)	12m 38s
	1h 24m 28s

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