README.ASIC: Clarify syntax of --scan-serial usage for USB Erupters

README.ASIC

@@ -82,9 +82,11 @@ BLOCK ERUPTER USB
 -----------------
 
 These will autodetect if supported by the device; otherwise, you need to use
-the '--scan-serial erupter:*' option to tell BFGMiner what device to probe. They
-communicate with the Icarus protocol, which has some additional options in
-README.FPGA
+the '--scan-serial erupter:<device>' option to tell BFGMiner what device to
+probe; if you know you have no other serial devices, or only ones that can
+tolerate garbage, you can use '--scan-serial erupter:all' to probe all serial
+ports. They communicate with the Icarus protocol, which has some additional
+options in README.FPGA
 
 
 ---