Clarify cmdln arg comments
Clarify cmdln arg comments

Closes: #3750
Approved by: davidiw

aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,libra/libra,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra