ENH: IO FOs - moved documentation to doc sub-directory

src/postProcessing/functionObjects/field/fieldFunctionObjectsDoc.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.
-
-    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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-    OpenFOAM is distributed in the hope that it will be useful, but WITHOUT ANY
-    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 grpFieldFunctionObjects Field function objects
-@{
-    \ingroup grpFunctionObjects
-    This group contains field-based function objects
-
-    Function objects in this group are packaged into the
-    libfieldFunctionObjects.so library.
-@}
-
-\*---------------------------------------------------------------------------*/