docs(common/):add comments for complex funcs #3782

Job	Run time
Basic Checks	1m 34s
Unit Tests	23m 8s
Integration Tests (raft)	12m 40s
Integration Tests (pvtdata)	11m 44s
Integration Tests (pvtdatapurge e2e)	11m 58s
Integration Tests (ledger)	13m 56s
Integration Tests (lifecycle)	5m 30s
Integration Tests (smartbft)	20m 54s
Integration Tests (discovery gossip devmode pluggable)	7m 6s
Integration Tests (gateway idemix pkcs11 configtx configtxlator)	17m 43s
Integration Tests (sbe nwo msp)	8m 4s
	2h 14m 17s

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