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

src/util/MessageFlags.js

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