Clarify gapsize >= 0.

libraries/base/diag/src/vhdl/diag_block_gen.vhd

@@ -36,9 +36,10 @@
 --
 --   The samples_per_packet >= 2, because the in the p_comb state machine the
 --   eop cannot occur at the sop.
---
 --   The blocks_per_sync >= 1, so it is possible to have one block per sync
 --   interval.
+--   The gapsize >= 0, so it is possible to have blocks directly after
+--   eachother without emoty cylces.
 --
 --   The BG sync interval takes blocks_per_sync * (samples_per_packet +
 --   gapsize) clock cylces. However in case of back pressure from out_siso