ENH: Added documentation file for function objects

src/OpenFOAM/db/functionObjects/functionObject/functionObjectsDoc.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.
+
+    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
+    details.
+
+    You should have received a copy of the GNU General Public License along with
+    OpenFOAM.  If not, see <http://www.gnu.org/licenses/>.
+
+\defgroup functionObjects Function objects
+@{
+    This group contains function objects
+@}
+
+\*---------------------------------------------------------------------------*/