Merge bitcoin/bitcoin#30525: doc, rpc : `#30275` followups

fa2f26960ee084971ab09959b213a9b8104482e5 [rpc, fees]: add more detail on the fee estimation modes (ismaelsadeeq)
6e7e620864cc7a2f3c3b576588afe4d44dc394ec [doc]: add `30275` release notes (ismaelsadeeq)

Pull request description:

  This PR:
  1. Adds release notes for #30275
  2. Describe fee estimation modes in RPC help texts

ACKs for top commit:
  achow101:
    ACK fa2f26960ee084971ab09959b213a9b8104482e5
  glozow:
    ACK fa2f26960ee084971ab09959b213a9b8104482e5
  willcl-ark:
    ACK fa2f26960ee
  tdb3:
    re ACK fa2f26960ee084971ab09959b213a9b8104482e5

Tree-SHA512: b8ea000b599297b954dc770137c29b47153e68644c58550a73e34b74ecb8b65e78417875481efdfdf6aab0018a9cd1d90d8baa5a015e70aca0975f6e1dc9598c

2 files changed, 30 insertions, 0 deletions

diff --git a/src/common/messages.cpp b/src/common/messages.cpp
index 6a3ebbca67..5fe3e9e4d8 100644
--- a/src/common/messages.cpp
+++ b/src/common/messages.cpp
@@ -53,6 +53,34 @@ const std::vector<std::pair<std::string, FeeEstimateMode>>& FeeModeMap()
     return FEE_MODES;
 }
 
+std::string FeeModeInfo(const std::pair<std::string, FeeEstimateMode>& mode, std::string& default_info)
+{
+    switch (mode.second) {
+        case FeeEstimateMode::UNSET:
+            return strprintf("%s means no mode set (%s). \n", mode.first, default_info);
+        case FeeEstimateMode::ECONOMICAL:
+            return strprintf("%s estimates use a shorter time horizon, making them more\n"
+                   "responsive to short-term drops in the prevailing fee market. This mode\n"
+                   "potentially returns a lower fee rate estimate.\n", mode.first);
+        case FeeEstimateMode::CONSERVATIVE:
+            return strprintf("%s estimates use a longer time horizon, making them\n"
+                   "less responsive to short-term drops in the prevailing fee market. This mode\n"
+                   "potentially returns a higher fee rate estimate.\n", mode.first);
+        default:
+            // Other modes apart from the ones handled are fee rate units; they should not be clarified.
+            assert(false);
+    }
+}
+
+std::string FeeModesDetail(std::string default_info)
+{
+    std::string info;
+    for (const auto& fee_mode : FeeModeMap()) {
+        info += FeeModeInfo(fee_mode, default_info);
+    }
+    return strprintf("%s \n%s", FeeModes(", "), info);
+}
+
 std::string FeeModes(const std::string& delimiter)
 {
     return Join(FeeModeMap(), delimiter, [&](const std::pair<std::string, FeeEstimateMode>& i) { return i.first; });
diff --git a/src/common/messages.h b/src/common/messages.h
index 68e7bb2169..5827fccee0 100644
--- a/src/common/messages.h
+++ b/src/common/messages.h
@@ -26,6 +26,8 @@ enum class PSBTError;
 bool FeeModeFromString(const std::string& mode_string, FeeEstimateMode& fee_estimate_mode);
 std::string StringForFeeReason(FeeReason reason);
 std::string FeeModes(const std::string& delimiter);
+std::string FeeModeInfo(std::pair<std::string, FeeEstimateMode>& mode);
+std::string FeeModesDetail(std::string default_info);
 std::string InvalidEstimateModeErrorMessage();
 bilingual_str PSBTErrorString(PSBTError error);
 bilingual_str TransactionErrorString(const node::TransactionError error);