Add ddocumentation on MLNodeLinOp overset mask

Docs/sphinx_documentation/source/LinearSolvers.rst

@@ -293,6 +293,18 @@ For Robin boundary conditions, the ghost cells in
 store the numerical values in the condition,
 :math:`a\phi + b\frac{\partial\phi}{\partial n} = f`.
 
+4) Nodal solver provides the option to use an overset mask:
+
+.. highlight:: c++
+
+::
+
+   // omask is either 0 or 1. 1 means the node is an unknown. 0 means it's known.
+   void setOversetMask (int amrlev, const iMultiFab& a_dmask);
+
+Note this is an integer (not bool) MultiFab, so the values must be only either 0 or 1.
+
+
 .. _sec:linearsolver:pars:
 
 Parameters