Document the api layer's operational models, and fix two headers that had gone stale against the code

Plugin/unraid/api/watchdog.php

@@ -5,6 +5,16 @@
 //   resource, docker, system, storage, network and stability — together with the thresholds
 //   each one is judging against, for the watchdog tab's poll.
 //
+// OPERATIONAL MODEL
+//   Served from a 5-minute cache unless ?live is present. Assembling this payload reads every
+//   watchdog's state files and resolves every threshold out of master.conf, which is far more
+//   work than the tab's poll needs — the watchdogs themselves only run every 15 minutes, so a
+//   fresher answer would describe the same cycle.
+//
+//   The cache is consulted before include/watchdog.php is even loaded, so a cache hit costs one
+//   file read and nothing else. ?live skips the read, recomputes, and writes the result back, so
+//   an explicit refresh also benefits the next visitor rather than being discarded.
+//
 // DESIGN PRINCIPLES
 //   Thin transport. State-file parsing and threshold resolution live in
 //   include/watchdog.php; this file only sets the content type and encodes.
@@ -15,7 +25,9 @@
 //       against, so the page never has to fetch the two independently and risk mismatching
 //       them across a conf edit.
 //
-//   No parameters. Which watchdogs exist is fixed by the codebase, not by the request.
+//   The only parameter is a freshness override. ?live decides how old an answer may be, never
+//   what is in it — which watchdogs exist is fixed by the codebase, and no request can select,
+//   filter or widen the set.
 //
 // OPERATIONAL SAFEGUARDS
 //   Read-only. Nothing here clears a strike, lifts a skip-list entry, restarts a container,