Clarify blocks_per_sync >= 1 and duration of BG sync interval.

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

@@ -37,6 +37,13 @@
 --   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 BG sync interval takes blocks_per_sync * (samples_per_packet +
+--   gapsize) clock cylces. However in case of back pressure from out_siso
+--   the BG sync interval become longer, due to clock cycles with valid = '0'.
+--
 --   The MM reading starts at mem_low_adrs when the BG is first enabled. If
 --   the mem_high_adrs-mem_low_adrs+1 < samples_per_packet then the reading
 --   wraps and continues from mem_low_adrs. For every new block the reading