ENH: forces FO - moved documentation to doc sub-directory

src/postProcessing/functionObjects/forces/forcesFunctionObjectsDoc.H

-/*---------------------------------------------------------------------------*\
-  =========                 |
-  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
-   \\    /   O peration     |
-    \\  /    A nd           | Copyright (C) 2012 OpenFOAM Foundation
-     \\/     M anipulation  |
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-License
-    This file is part of OpenFOAM.
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-    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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-
-\defgroup grpForcesFunctionObjects Forces function objects
-@{
-    \ingroup grpFunctionObjects
-    This group contains force-based function objects
-
-    Function objects in this group are packaged into the
-    libforcesFunctionObjects.so library.
-@}
-
-\*---------------------------------------------------------------------------*/