message: [Sapling] Add comment about size calculations for converted serialized keys.
  Coming from upstream
diff: @@ -38,6 +38,7 @@ public:
  // ConvertBits requires unsigned char, but CDataStream uses char
  std::vector<unsigned char> seraddr(ss.begin(), ss.end());
  std::vector<unsigned char> data;
  + // See calculation comment below
  data.reserve((seraddr.size() * 8 + 4) / 5);
  ConvertBits<8, 5, true>([&](unsigned char c) { data.push_back(c); }, seraddr.begin(), seraddr.end());
  return bech32::Encode(m_params.Bech32HRP(CChainParams::SAPLING_PAYMENT_ADDRESS), data);
  @@ -67,6 +68,7 @@ public:
  // ConvertBits requires unsigned char, but CDataStream uses char
  std::vector<unsigned char> serkey(ss.begin(), ss.end());
  std::vector<unsigned char> data;
  + // See calculation comment below
  data.reserve((serkey.size() * 8 + 4) / 5);
  ConvertBits<8, 5, true>([&](unsigned char c) { data.push_back(c); }, serkey.begin(), serkey.end());
  std::string ret = bech32::Encode(m_params.Bech32HRP(CChainParams::SAPLING_SPENDING_KEY), data);
  @@ -80,6 +82,10 @@ public:
  namespace KeyIO {
  + // Sizes of SaplingPaymentAddress and SaplingSpendingKey after
  + // ConvertBits<8, 5, true>(). The calculations below take the
  + // regular serialized size in bytes, convert to bits, and then
  + // perform ceiling division to get the number of 5-bit clusters.
  const size_t ConvertedSaplingPaymentAddressSize = ((32 + 11) * 8 + 4) / 5;
  const size_t ConvertedSaplingSpendingKeySize = (32 * 8 + 4) / 5;
  