adb: get_balance: add comment about #8835

> These kind of checks look incorrect and confused/confusing for me:
> https://github.com/spesmilo/electrum/blob/43a5d034268e4bb6e3dfa3db3fa632642c154d68/electrum/gui/qml/qeinvoice.py#L388
> https://github.com/spesmilo/electrum/blob/43a5d034268e4bb6e3dfa3db3fa632642c154d68/electrum/gui/qt/main_window.py#L1801

> For example, consider creating a local tx that spends all UTXOs in the wallet, and consolidates them to another ismine address.
> Such a tx basically does not change what wallet.get_balance() returns.
> but to the checks listed above, the confirmed balance of the wallet I guess should be 0?

electrum/address_synchronizer.py

@@ -860,6 +860,7 @@ class AddressSynchronizer(Logger, EventListener):
                     excluded_coins: Set[str] = None) -> Tuple[int, int, int]:
         """Return the balance of a set of addresses:
         confirmed and matured, unconfirmed, unmatured
+        Note: intended for display-purposes. would need extreme care for "has enough funds" checks (see #8835)
         """
         if excluded_addresses is None:
             excluded_addresses = set()