Merge pull request arfc#275 from nglaser3/add_doc

Added documentation to NtAction

doc/content/source/actions/NtAction.md

@@ -1,20 +1,42 @@
 # NtAction
 
-!alert construction title=Undocumented Action Class
-The NtAction has not been documented. The content listed below should be used as a starting point for
-documenting the class, which includes the typical automatic documentation associated with an Action;
-however, what is contained is ultimately determined by what is necessary to make the documentation
-clear for users.
+!alert note
+This action sets up ONLY the variables, kernels, and BCs for neutron diffusion,
+NOT the variables, kernels, and BCs for precursor tracking.
+To generate these, see [PrecursorAction](PrecursorAction.md).
 
 !syntax description /Nt/NtAction
 
 ## Overview
 
-!! Replace these lines with information regarding the NtAction action.
+```NtAction``` greatly reduces input file length and complexity by automatically
+setting up variables, kernels, and BCs for the neutron diffusion equations.
+When including only the required input parameters, this action constructs the
+neutron group variables and their associated kernels:
+[SigmaR](SigmaR.md), [GroupDiffusion](GroupDiffusion.md),
+[InScatter](InScatter.md), and [CoupledFissionKernel](CoupledFissionKernel.md).
+With ```account_delayed``` set to ```True```,
+[DelayedNeutronSource](DelayedNeutronSource.md) is also constructed,
+otherwise this kernel is not constructed and the simulation does not consider
+the delayed neutron precursor effects on the neutron flux distributions.
+
+For more information regarding the use of ```NtAction``` please refer to the
+tutorials located [here](tutorials.md), specifically the +Multiphysics Reactor
+Simulations+ section.
 
 ## Example Input File Syntax
 
-!! Describe and include an example of how to use the NtAction action.
+An example input file without the ```NtAction```, showing only the portion
+affected by the action:
+
+!listing tutorial/eigenvalue/nts.i
+	start=Variables end=Materials remove=Precursors
+
+
+And then the same information created with the ```NtAction```:
+
+!listing tutorial/eigenvalue/nts-action.i
+	block=Nt
 
 !syntax description /Nt/NtAction