values smaller than 0.001 sat/B can't be represented by CFeeRate, but they are not rejected.

```
./src/bitcoin-cli -named sendtoaddress address=bcrt1qwkmhqum095zau5rf2velq55cq938vrd7uw8m4r amount=1.23 fee_rate=0.0009

error code: -6
error message:
Fee rate (0.000 sat/vB) is lower than the minimum fee rate setting (1.000 sat/vB)
```

```
./src/bitcoin-cli -named sendtoaddress address=bcrt1qwkmhqum095zau5rf2velq55cq938vrd7uw8m4r amount=1.23 fee_rate=0.000000000009

error code: -3
error message:
Invalid amount

{
    std::string result;
    for (const auto& r : m_results) {
        if (r.m_type == RPCResult::Type::ANY) continue; 
        if (r.m_cond.empty()) {
            result += "\nResult:\n";
        } else {
            result += "\nResult (" + r.m_cond + "):\n";
        }
        Sections sections;
        r.ToSections(sections);
        result += sections.ToString();
    }
    return result;
}
std::string RPCExamples::ToDescriptionString() const
{
    return m_examples.empty() ? m_examples : "\nExamples:\n" + m_examples;
}
UniValue RPCHelpMan::HandleRequest(const JSONRPCRequest& request) const