[handbook] Update dune-subgrid description in 2_detailedinstall.tex

doc/handbook/2_detailedinstall.tex

@@ -287,7 +287,8 @@ and some more libraries and tools which are prerequisites for their use.
   
 \item \textbf{dune-subgrid}: The dune-subgrid module allows to mark elements of another hierarchical dune grid.
 The set of marked elements can then be accessed as a hierarchical dune grid in its own right.
-Dune-Subgrid provides the full grid interface including adaptive mesh refinement.
+Dune-Subgrid provides the full grid interface including adaptive mesh refinement
+and supports periodic boundary conditions.
   Download: \url{https://git.imp.fu-berlin.de/agnumpde/dune-subgrid.git}
   
 \item \textbf{dune-spgrid}: The DUNE module dune-spgrid provides a structured, parallel grid.