Merge bitcoin/bitcoin#21752: doc: Clarify that feerates are per virtual size

fae196147b doc: Clarify that feerates are per virtual size (MarcoFalke)
fa83e95ac6 scripted-diff: Clarify that feerates are per virtual size (MarcoFalke)

Pull request description:

  By implementing segwit, it is already clear that all feerates in Bitcoin Core are denoted in (amount/virtual size). Though, there is  inconsistency, as some places use kvB, some use kB. Thus, replace all with "kvB".

  See also commit 6da3afbaee, which did the replacement for wallet RPCs.

ACKs for top commit:
  ryanofsky:
    Code review ACK fae196147b. Checked instances where units were being added in the second commit and they all looked right.

Tree-SHA512: ab70d13cde7d55c1ac931bddc2b45aa218fc75ef46cb6ea9e5a30b1d4dbf27889c2b6357299a6c5427912443a46ec3592a4809dae335e03162bd2120a0f7f8ad

src/qt/forms/sendcoinsdialog.ui

@@ -850,7 +850,7 @@
                    <property name="toolTip">
                     <string>Specify a custom fee per kB (1,000 bytes) of the transaction's virtual size.
 
-Note:  Since the fee is calculated on a per-byte basis, a fee of "100 satoshis per kB" for a transaction size of 500 bytes (half of 1 kB) would ultimately yield a fee of only 50 satoshis.</string>
+Note:  Since the fee is calculated on a per-byte basis, a fee rate of "100 satoshis per kvB" for a transaction size of 500 virtual bytes (half of 1 kvB) would ultimately yield a fee of only 50 satoshis.</string>
                    </property>
                    <property name="text">
                     <string>per kilobyte</string>

src/qt/sendcoinsdialog.cpp

@@ -736,7 +736,7 @@ void SendCoinsDialog::updateFeeMinimizedLabel()
     if (ui->radioSmartFee->isChecked())
         ui->labelFeeMinimized->setText(ui->labelSmartFee->text());
     else {
-        ui->labelFeeMinimized->setText(BitcoinUnits::formatWithUnit(model->getOptionsModel()->getDisplayUnit(), ui->customFee->value()) + "/kB");
+        ui->labelFeeMinimized->setText(BitcoinUnits::formatWithUnit(model->getOptionsModel()->getDisplayUnit(), ui->customFee->value()) + "/kvB");
     }
 }
 
@@ -771,7 +771,7 @@ void SendCoinsDialog::updateSmartFeeLabel()
     FeeReason reason;
     CFeeRate feeRate = CFeeRate(model->wallet().getMinimumFee(1000, *m_coin_control, &returned_target, &reason));
 
-    ui->labelSmartFee->setText(BitcoinUnits::formatWithUnit(model->getOptionsModel()->getDisplayUnit(), feeRate.GetFeePerK()) + "/kB");
+    ui->labelSmartFee->setText(BitcoinUnits::formatWithUnit(model->getOptionsModel()->getDisplayUnit(), feeRate.GetFeePerK()) + "/kvB");
 
     if (reason == FeeReason::FALLBACK) {
         ui->labelSmartFee2->show(); // (Smart fee not initialized yet. This usually takes a few blocks...)