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

src/util/Speaking.js

@@ -9,6 +9,13 @@ const BitField = require('./BitField');
  */
 class Speaking extends BitField {}
 
+/**
+ * @name Speaking
+ * @kind constructor
+ * @memberof Speaking
+ * @param {BitFieldResolvable} [bits=0] Bit(s) to read from
+ */
+
 /**
  * Numeric speaking flags. All available properties:
  * * `SPEAKING`