Add a couple comments pointing to particular sections of RFC6979.
Add a couple comments pointing to particular sections of RFC6979.

sipa/secp256k1,cddjr/BitcoinUnlimited,01BTC10/secp256k1,chfast/secp256k1,martindale/secp256k1-zkp,BTCfork/hardfork_prototype_1_mvf-bu,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,evoskuil/secp256k1,libbitcoin/secp256k1,GSongHashrate/secp256k1,afk11/secp256k1,pmienk/secp256k1,Bitcoin-com/BUcash,randy-waterhouse/secp256k1,shea256/secp256k1-zkp,evoskuil/secp256k1,instagibbs/secp256k1,01BTC10/secp256k1,marcusdiaz/BitcoinUnlimited,GSongHashrate/secp256k1,Gustav-Simonsson/secp256k1,digitalbitbox/secp256k1,vlajos/secp256k1,afk11/secp256k1,BTCfork/hardfork_prototype_1_mvf-bu,Justaphf/BitcoinUnlimited,sipa/secp256k1-zkp,voisine/secp256k1,GSongHashrate/secp256k1,pmienk/secp256k1,Justaphf/BitcoinUnlimited,digitalbitbox/secp256k1,martindale/secp256k1-zkp,rustyrussell/secp256k1,Bitcoin-com/BUcash,apoelstra/secp256k1,krzysztofwos/BitcoinUnlimited,vlajos/secp256k1,apoelstra/secp256k1,ElementsProject/secp256k1-zkp,Gustav-Simonsson/secp256k1,libbitcoin/secp256k1,bitcoin-core/secp256k1,libmetrocoin/secp256k1,01BTC10/secp256k1,BitcoinUnlimited/BitcoinUnlimited,Bitcoin-com/BUcash,BitcoinUnlimited/BitcoinUnlimited,kleetus/secp256k1,Justaphf/BitcoinUnlimited,randy-waterhouse/secp256k1,ChrisXin/secp256k1,sipa/secp256k1-zkp,cddjr/BitcoinUnlimited,marcusdiaz/BitcoinUnlimited,krzysztofwos/BitcoinUnlimited,peterdettman/secp256k1,kleetus/secp256k1,apoelstra/secp256k1,martindale/secp256k1,digitalbitbox/secp256k1,sipa/secp256k1-zkp,BitcoinUnlimited/BitcoinUnlimited,evoskuil/secp256k1,Justaphf/BitcoinUnlimited,afk11/secp256k1,bitcoin/secp256k1,marcusdiaz/BitcoinUnlimited,Bitcoin-com/BUcash,cddjr/BitcoinUnlimited,shea256/secp256k1-zkp,bitcoin/secp256k1,bitcoin/secp256k1,BitcoinUnlimited/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,apoelstra/secp256k1,pmienk/secp256k1,voisine/secp256k1,martindale/secp256k1,voisine/secp256k1,ElementsProject/secp256k1-zkp,peterdettman/secp256k1,randy-waterhouse/secp256k1,martindale/secp256k1,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,afk11/secp256k1,Gustav-Simonsson/secp256k1,shea256/secp256k1-zkp,digitalbitbox/secp256k1,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,sipa/secp256k1,cddjr/BitcoinUnlimited,bitcoin-core/secp256k1,voisine/secp256k1,bitcoin/secp256k1,ChrisXin/secp256k1,vlajos/secp256k1,martindale/secp256k1-zkp,rustyrussell/secp256k1,krzysztofwos/BitcoinUnlimited,voisine/secp256k1,libmetrocoin/secp256k1,ChrisXin/secp256k1,BTCfork/hardfork_prototype_1_mvf-bu,shea256/secp256k1-zkp,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,chfast/secp256k1,01BTC10/secp256k1,krzysztofwos/BitcoinUnlimited,kleetus/secp256k1,instagibbs/secp256k1,libmetrocoin/secp256k1,Justaphf/BitcoinUnlimited,GSongHashrate/secp256k1,BTCfork/hardfork_prototype_1_mvf-bu,libbitcoin/secp256k1,instagibbs/secp256k1,libmetrocoin/secp256k1,marcusdiaz/BitcoinUnlimited,instagibbs/secp256k1,bitcoin/secp256k1,marcusdiaz/BitcoinUnlimited,rustyrussell/secp256k1,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,Bitcoin-com/BUcash,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,rustyrussell/secp256k1,pmienk/secp256k1,martindale/secp256k1-zkp,kleetus/secp256k1,Bitcoin-com/BUcash,pmienk/secp256k1,digitalbitbox/secp256k1,kleetus/secp256k1,vlajos/secp256k1,chfast/secp256k1,martindale/secp256k1,evoskuil/secp256k1,sipa/secp256k1-zkp,chfast/secp256k1,sipa/secp256k1-zkp,krzysztofwos/BitcoinUnlimited,BTCfork/hardfork_prototype_1_mvf-bu,libbitcoin/secp256k1,cddjr/BitcoinUnlimited,randy-waterhouse/secp256k1,cddjr/BitcoinUnlimited,BTCfork/hardfork_prototype_1_mvf-bu,libbitcoin/secp256k1,sipa/secp256k1,afk11/secp256k1,sipa/secp256k1,krzysztofwos/BitcoinUnlimited,Justaphf/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,marcusdiaz/BitcoinUnlimited,instagibbs/secp256k1,evoskuil/secp256k1,chfast/secp256k1,ChrisXin/secp256k1,apoelstra/secp256k1,Gustav-Simonsson/secp256k1