docs(BitField): document constructors of deriving classes (#3903)

src/util/ActivityFlags.js

@@ -8,6 +8,13 @@ const BitField = require('./BitField');
  */
 class ActivityFlags extends BitField {}
 
+/**
+ * @name ActivityFlags
+ * @kind constructor
+ * @memberof ActivityFlags
+ * @param {BitFieldResolvable} [bits=0] Bit(s) to read from
+ */
+
 /**
  * Numeric activity flags. All available properties:
  * * `INSTANCE`