Add documentation on the use of types::global_dof_index.

git-svn-id: https://svn.dealii.org/branches/branch_bigger_global_dof_indices_4@29320 0785d39b-7218-0410-832d-ea1e28bc413d

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+//-------------------------------------------------------------------------
+//    $Id: coding_conventions.h 29312 2013-04-17 13:00:07Z turcksin $
+//    Version: $Name$
+//
+//    Copyright (C) 2013 by the deal.II authors
+//
+//    This file is subject to QPL and may not be  distributed
+//    without copyright and license information. Please refer
+//    to the file deal.II/doc/license.html for the  text  and
+//    further information on this license.
+//
+//-------------------------------------------------------------------------
+
+/**
+ * @page GlobalDoFIndex When to use types::global_dof_index instead of
+ * unsigned int
+ *
+ * When the 64-bits version of deal.II is used, it becomes necessary to
+ * declare as types::global_dof_index (unsigned long long int) instead of
+ * unsigned int. Here, we want to clarify when types::global_dof_index must be
+ * used.
+ *
+ * <dl>
+ *
+ * <dt class="glossary">@anchor GlobalDoFIndexBlockIndices
+ * <b>BlockIndices</b></dt>
+ * <dd>
+ * The number of blocks is an unsigned int because the number is expected to
+ * be low, i.e less than four billions. However, the size of the block is a
+ * types::global_dof_index because each block can be arbitrary large.
+ * </dd>
+ *
+ * <dt class="glossary">@anchor GlobalDoFIndexCell <b>Cell</b></dt>
+ * <dd>
+ * The ID of cell is not unique. Cells with different levels of refinement
+ * and/or on different processors can have the same ID. Thus, all the data
+ * associated to cells can be unsigned int.
+ * </dd>
+ *
+ * <dt class="glossary">@anchor GlobalDoFIndexDoFHandler
+ * <b>DoFHandler</b></dt>
+ * <dd>
+ * The ID of each degree of freedom is unique. Therefore, degrees of freedom
+ * are types::global_dof_index.
+ * </dd>
+ *
+ * <dt class="glossary">@anchor GlobalDoFIndexFullMatrix
+ * <b>FullMatrix</b></dt>
+ * <dd>
+ * The numbers of row and column are types::global_dof_index even if it is not
+ * expected that someone will create a FullMatrix with so many entries.
+ * However, ConstraintMatrix is templated on the matrix type and thus, the
+ * size of a FullMatrix has to be of the same type than the size of
+ * SparseMatrix.
+ * </dd>
+ *
+ * <dt class="glossary">@anchor GlobalDoFIndexSparseMatrix
+ * <b>SparseMatrix</b></dt>
+ * <dd>
+ * The size SparseMatrix can be arbitrary large therefore,
+ * types::global_do_index is used.
+ */