ADD: Document Fill action in MBO

Author: Zach Banks

include/databento/enums.hpp

@@ -132,6 +132,8 @@ enum Action : char {
   Modify = 'M',
   // A trade executed.
   Trade = 'T',
+  // An existing order was filled.
+  Fill = 'F',
   // An order was cancelled.
   Cancel = 'C',
   // A new order was added.

src/enums.cpp

@@ -300,6 +300,9 @@ const char* ToString(Action action) {
     case Action::Trade: {
       return "Trade";
     }
+    case Action::Fill: {
+      return "Fill";
+    }
     case Action::Cancel: {
       return "Cancel";
     }