Added documentation for openmp parameter.

Documentation/source/site-specific-config.rst

@@ -148,6 +148,18 @@ rsync, ar, ...).
     tool_box = ToolBox()
     default_c_compiler = tool_box.get_tool(Category.C_COMPILER)
 
+There is special handling for compilers and linkers: the build
+configuration stores the information if an MPI and/or OpenMP build
+is requested. So when a default tool is requested by the ToolBox
+from the ToolRepository (i.e. when the user has not added specific
+compilers or linkers), this information is taken into account, and
+only a compiler that will fulfil the requirements is returned. For
+example, if you have `gfortran` and `mpif90-gfortran` defined in this
+order in the ToolRepository, and request the default compiler for an
+MPI build, the `mpif90-gfortran` instance is returned, not `gfortran`.
+On the other hand, if no MPI is requested, an MPI-enabled compiler
+might be returned, which does not affect the final result, since
+an MPI compiler just adds include- and library-paths.
 
 TODO
 ====

source/fab/build_config.py

@@ -56,9 +56,13 @@ def __init__(self, project_label: str,
             created from this name, with spaces replaced by underscores.
         :param tool_box: The ToolBox with all tools to use in the build.
         :param mpi: whether the project uses MPI or not. This is used to
-            pick a default compiler (if not explicitly set in the ToolBox),
-            and controls PSyclone parameters.
-        :param openmp: whether the project should use OpenMP or not.
+            pick a default compiler (if none is explicitly set in the
+            ToolBox), and controls PSyclone parameters.
+        :param openmp: as with `mpi`, this controls whether the project is
+            using OpenMP or not. This is used to pick a default compiler
+            (if none is explicitly set in the ToolBox). The compiler-specific
+            flag to enable OpenMP will automatically be added when compiling
+            and linking.
         :param multiprocessing:
             An option to disable multiprocessing to aid debugging.
         :param n_procs: