ENH: Added documentation file for incompressible wall functions

src/turbulenceModels/incompressible/RAS/derivedFvPatchFields/wallFunctions/incompressibleWallFunctionsDoc.H

+/*---------------------------------------------------------------------------*\
+  =========                 |
+  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
+   \\    /   O peration     |
+    \\  /    A nd           | Copyright (C) 2012 OpenFOAM Foundation
+     \\/     M anipulation  |
+-------------------------------------------------------------------------------
+License
+    This file is part of OpenFOAM.
+
+    OpenFOAM is free software: you can redistribute it and/or modify it under
+    the terms of the GNU General Public License as published by the Free
+    Software Foundation, either version 3 of the License, or (at your option)
+    any later version.
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+    WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+    FOR A PARTICULAR PURPOSE.  See the GNU General Public License for more
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+    You should have received a copy of the GNU General Public License along with
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+
+\defgroup icoWallFunctions RAS incompressible wall functions
+@{
+    \ingroup boundaryConditions
+    This group contains incompressible RAS turbulence model wall functions
+@}
+
+\*---------------------------------------------------------------------------*/