doc: explain what a JEDEC ID is

The bscan_jedec command and tutorial referenced the JEDEC ID without
defining it. Describe the 0x9F RDID command and the manufacturer +
device byte layout, in the tutorial and the command help.

modules/script/script.c

@@ -2954,6 +2954,8 @@ static int cmd_bscan_load_bitstream(script_ctx *ctx, char *line)
 const char *cmd_bscan_jedec_help[] = {
     "<device>",
     "Read the SPI flash JEDEC ID (0x9F) through the loaded BSCAN proxy.",
+    "JEDEC ID = 1 manufacturer byte (e.g. 0x20 Micron) + 2 device bytes;",
+    "identifies which flash chip is wired to the FPGA.",
     "Requires a proxy bitstream already loaded (bscan_load_bitstream).",
     ""};
 static int cmd_bscan_jedec(script_ctx *ctx, char *line)