Doc: Docstring remaining mli files in lib_protocol

As a follow-up from MR !2628 (merged) and a sub-issue of #205.

Several interfaces under src/proto_alpha/lib_protocol are still not documented with simple docstrings.

• bootstrap_storage.mli
• commitment_repr.mli
• constants_storage.mli
• contract_services.mli
• cycle_repr.mli
• delegate_activation_storage.mli
• delegate_services.mli
• frozen_deposits_storage.mli
• michelson_v1_gas.mli
• nonce_storage.mli
• roll_repr_legacy.mli
• script_map.mli
• script_set.mli
• script_tc_errors_registration.mli
• stake_storage.mli
• voting_services.mli