docs(common/):add comments for complex funcs #3781

Job	Run time
Basic Checks	1m 42s
Unit Tests	21m 51s
Integration Tests (raft)	13m 12s
Integration Tests (pvtdata)	12m 9s
Integration Tests (pvtdatapurge e2e)	11m 26s
Integration Tests (ledger)	13m 3s
Integration Tests (lifecycle)	6m 6s
Integration Tests (smartbft)	20m 46s
Integration Tests (discovery gossip devmode pluggable)	6m 51s
Integration Tests (gateway idemix pkcs11 configtx configtxlator)	17m 35s
Integration Tests (sbe nwo msp)	8m 34s
	2h 13m 15s

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