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

src/util/SystemChannelFlags.js

@@ -10,6 +10,13 @@ const BitField = require('./BitField');
  */
 class SystemChannelFlags extends BitField {}
 
+/**
+ * @name SystemChannelFlags
+ * @kind constructor
+ * @memberof SystemChannelFlags
+ * @param {SystemChannelFlagsResolvable} [bits=0] Bit(s) to read from
+ */
+
 /**
  * Data that can be resolved to give a sytem channel flag bitfield. This can be:
  * * A string (see {@link SystemChannelFlags.FLAGS})