Adding additional references to the grain growth documentation page in the phase field module

[tonkmr]

Reason

Professor LQ Chen reached out and correctly pointed out that currently we reference the Moelans [2008] paper for our grain growth model, but that that paper actually pulled the model from his earlier publications.

Design

He has asked that we reference two of his papers as well:
Chen, L.Q. And W. Yang, Computer-Simulation Of The Domain Dynamics Of A Quenched System With A Large Number Of Nonconserved Order Parameters - The Grain-Growth Kinetics. Physical Review B, 1994. 50(21): P. 15752-15756.
Chen, L.Q., A Novel Computer-Simulation Technique For Modeling Grain-Growth. Scripta Metallurgica Et Materialia, 1995. 32(1): P. 115-120.

Impact

This is a very minor change. It is only adding two references to a documentation page. There will be no impacts on anything else.

[tonkmr]

@dschwen @laagesen This is a minor change, but I am swamped right now. Could someone at INL make this change? Thanks!

[laagesen]

Thanks for bringing this up @tonkmr, we will get it added to the doco