add documentation

cpp/kspace.hpp

@@ -39,6 +39,17 @@
 enum field_backend {FFTW};
 enum kspace_dealias_type {TWO_THIRDS, SMOOTH};
 
+/** \brief A class for handling Fourier representation tasks.
+ *
+ *      - contains wavenumber information (specific to each MPI process).
+ *      This includes values of kx, ky, kz, including lowest modes dkx etc,
+ *      as well as number of modes within sferical shells and mean wavenumber
+ *      within shells.
+ *      - has methods for spectrum computation and similar.
+ *      - has methods for filtering.
+ *      - has CLOOP methods, useful for computing arbitrary formulas over the
+ *      Fourier space grid (i.e. use lambda expressions).
+ */
 
 template <field_backend be,
           kspace_dealias_type dt>