Add field grouping ScalaDoc for ArbiterIO (#2208)

* Update Arbiter.scala

* Update src/main/scala/chisel3/util/Arbiter.scala

changed group name

Co-authored-by: Megan Wachs <megan@sifive.com>

* minor changes on grouping ArbiterIO

* removed unmatched closing brace

* Remove groupdesc from Arbiter.scala

* Added groupdesc to Aggregate.scala

* Update Arbiter.scala

* Update core/src/main/scala/chisel3/Aggregate.scala

Co-authored-by: Megan Wachs <megan@sifive.com>

* Update Arbiter.scala

* Update src/main/scala/chisel3/util/Arbiter.scala

Added suugestions.

Co-authored-by: Megan Wachs <megan@sifive.com>

* added suggestions from review

* added suggestions from review

* Resolved conflicts

* update Arbiter.scala

* Update core/src/main/scala/chisel3/Aggregate.scala

deleted groudesc for ArbiterIO

Co-authored-by: Megan Wachs <megan@sifive.com>

* Update Scaladoc syntax

* removed some lines

* Better documentation

* Removed @param and @gen

* Update core/src/main/scala/chisel3/Aggregate.scala

Co-authored-by: Megan Wachs <megan@sifive.com>

* Update src/main/scala/chisel3/util/Arbiter.scala

Co-authored-by: Megan Wachs <megan@sifive.com>

* Added groupdesc to ArbiterIO

* Update src/main/scala/chisel3/util/Arbiter.scala

Co-authored-by: Megan Wachs <megan@sifive.com>

* Update core/src/main/scala/chisel3/Aggregate.scala

Co-authored-by: Megan Wachs <megan@sifive.com>

* Update Arbiter.scala

* Update src/main/scala/chisel3/util/Arbiter.scala

Co-authored-by: Megan Wachs <megan@sifive.com>

* Update Arbiter.scala

Co-authored-by: Megan Wachs <megan@sifive.com>