docs(common/):add comments for complex funcs #3784

Job	Run time
Basic Checks	1m 47s
Unit Tests	22m 55s
Integration Tests (raft)	11m 51s
Integration Tests (pvtdata)	12m 36s
Integration Tests (pvtdatapurge e2e)	10m 49s
Integration Tests (ledger)	14m 23s
Integration Tests (lifecycle)	7m 7s
Integration Tests (smartbft)	20m 5s
Integration Tests (discovery gossip devmode pluggable)	6m 43s
Integration Tests (gateway idemix pkcs11 configtx configtxlator)	20m 36s
Integration Tests (sbe nwo msp)	8m 34s
	2h 17m 26s

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