Document the class

[[Imported from SVN: r3598]]

dune/solvers/common/canignore.hh

 #ifndef CAN_IGNORE_HH
 #define CAN_IGNORE_HH
 
+/** \brief Abstract base class for solvers and iterationsteps that are
+ *      able to ignore degrees of freedom described by a bit field.
+ */
 template <class BitVectorType>
 class CanIgnore
 {