e1608: probe and docs for the network scan

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.md

@@ -117,7 +117,13 @@ only when `UDPSCOPE_PROBE_SOURCE_KINDS` is set, and exists only for the self-che
 
 The second built-in kind is the **MCC E-1608** Ethernet DAQ (8 single-ended or 4 differential 16-bit inputs, two
 analog outputs, eight digital lines and a 32-bit counter). Its connection form takes an address, or a MAC address
-to look up (optionally with the subnet to sweep), and groups the acquisition settings (scan rate, external clock,
+to look up (optionally with the subnet to sweep). A **Scan network** row at the top of the form probes every
+network interface of the host for boards (about a second and a half), lists each one by address and MAC, and a
+click on a result fills the address, or **Use MAC** fills the MAC and leaves the address empty. Results are not
+saved: only the filled field is. A MAC with no subnet is now looked up on every interface, not only the default
+route's, because a single limited broadcast leaves through the default route alone and missed a board on another
+interface (found on a real board on 2026-11-02, where the board was on `net0` and the default route was
+`wlan0`). The form also groups the acquisition settings (scan rate, external clock,
 trigger, restart after an overrun, factory calibration, the digital poll rate), a range per analog input, and the
 direction and initial value of each digital line. The analog inputs are the plotted channels; each digital input
 and the counter is an extra channel only if you ask for it. Two side-stack tabs belong to it: **Analog out**