Added usage comment.

libraries/base/common/tb/vhdl/tb_common_complex_mult.vhd

@@ -18,6 +18,13 @@
 -- along with this program.  If not, see <http://www.gnu.org/licenses/>.
 --
 -------------------------------------------------------------------------------
+-- Purpose: Verify different architectures of common_complex_mult
+-- Description:
+--   p_verify verifies that the instances of common_complex_mult all yield the
+--   expected results and ASSERTs an ERROR in case they differ.
+-- Usage:
+-- > as 10
+-- > run -all  -- signal tb_end will stop the simulation by stopping the clk
 
 LIBRARY IEEE;
 USE IEEE.std_logic_1164.ALL;