Clarify c_fifo_size.

libraries/io/eth/src/vhdl/eth_tester_rx.vhd

@@ -63,6 +63,10 @@ ARCHITECTURE str OF eth_tester_rx IS
   CONSTANT c_nof_total_counts     : NATURAL := 3;  -- 0 = nof_sop, 1 = nof_valid, 2 = nof_crc_corrupt
 
   CONSTANT c_empty_w              : NATURAL := 2;  -- for 0, 1, 2, 3 empty octets per word
+
+  -- Rx FIFO size can be much less than rx_block_sz_max, because st_clk >
+  -- eth_clk rate, but with st level tx-rx loopback the Rx FIFO does need
+  -- rx_block_sz_max FIFO size.
   CONSTANT rx_block_sz_max        : NATURAL := c_eth_tester_rx_block_len_max / c_word_sz;  -- = 9004 / 4 = 2251
   CONSTANT c_fifo_size            : NATURAL := true_log_pow2(rx_block_sz_max);  -- = 4096 words