qt send tab: when clicking "Max", show tooltip explaining max amt

Beginner users often ask why they cannot send their full balance.
Hence, this intends to reduce support load.
In terms of UI, maybe there is a better way to do this but this was
easy to do and still a good first step IMHO.

electrum/gui/qt/main_window.py

@@ -39,7 +39,7 @@ import asyncio
 from typing import Optional, TYPE_CHECKING, Sequence, List, Union
 
 from PyQt5.QtGui import QPixmap, QKeySequence, QIcon, QCursor, QFont
-from PyQt5.QtCore import Qt, QRect, QStringListModel, QSize, pyqtSignal
+from PyQt5.QtCore import Qt, QRect, QStringListModel, QSize, pyqtSignal, QPoint
 from PyQt5.QtCore import QTimer
 from PyQt5.QtWidgets import (QMessageBox, QComboBox, QSystemTrayIcon, QTabWidget,
                              QMenuBar, QFileDialog, QCheckBox, QLabel,
@@ -1461,6 +1461,17 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, Logger):
         __, x_fee_amount = run_hook('get_tx_extra_fee', self.wallet, tx) or (None, 0)
         amount_after_all_fees = amount - x_fee_amount
         self.amount_e.setAmount(amount_after_all_fees)
+        # show tooltip explaining max amount
+        mining_fee = tx.get_fee()
+        mining_fee_str = self.format_amount_and_units(mining_fee)
+        msg = _("Mining fee: {} (can be adjusted on next screen)").format(mining_fee_str)
+        if x_fee_amount:
+            twofactor_fee_str = self.format_amount_and_units(x_fee_amount)
+            msg += "\n" + _("2fa fee: {} (for the next batch of transactions)").format(twofactor_fee_str)
+        frozen_bal = self.get_frozen_balance_str()
+        if frozen_bal:
+            msg += "\n" + _("Some coins are frozen: {} (can be unfrozen in the Addresses or in the Coins tab)").format(frozen_bal)
+        QToolTip.showText(self.max_button.mapToGlobal(QPoint(0, 0)), msg)
 
     def get_contact_payto(self, key):
         _type, label = self.contacts.get(key)
@@ -1663,13 +1674,19 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, Logger):
 
     def get_text_not_enough_funds_mentioning_frozen(self) -> str:
         text = _("Not enough funds")
-        frozen_bal = sum(self.wallet.get_frozen_balance())
-        if frozen_bal:
-            text += " ({} {} {})".format(
-                self.format_amount(frozen_bal).strip(), self.base_unit(), _("are frozen")
+        frozen_str = self.get_frozen_balance_str()
+        if frozen_str:
+            text += " ({} {})".format(
+                frozen_str, _("are frozen")
             )
         return text
 
+    def get_frozen_balance_str(self) -> Optional[str]:
+        frozen_bal = sum(self.wallet.get_frozen_balance())
+        if not frozen_bal:
+            return None
+        return self.format_amount_and_units(frozen_bal)
+
     def pay_onchain_dialog(
             self, inputs: Sequence[PartialTxInput],
             outputs: List[PartialTxOutput], *,