Add some additional context for the sync pin error

When editing using synchronized pins mode, you place a pin on all units
but if you already have a pin that only exists on one unit, you might
get a confusing error.  This helps nudge people with to to resolve it.

Fixes https://gitlab.com/kicad/code/kicad/-/issues/18466

eeschema/tools/symbol_editor_pin_tool.cpp

@@ -232,6 +232,7 @@ bool SYMBOL_EDITOR_PIN_TOOL::PlacePin( SCH_COMMIT* aCommit, SCH_PIN* aPin )
                         test->GetUnit() );
 
             KIDIALOG dlg( m_frame, msg, _( "Confirmation" ), wxOK | wxCANCEL | wxICON_WARNING );
+            dlg.SetExtendedMessage( _( "Disable the 'Synchronize pins editing' option to avoid this message." ) );
             dlg.SetOKLabel( _( "Place Pin Anyway" ) );
             dlg.DoNotShowCheckbox( __FILE__, __LINE__ );