[Docs] (MG_Pipe, MobileGL): describe the bad-knob Fatal and the arm-time setting lookup

MobileGL/ConfigLoader.cpp

@@ -137,8 +137,9 @@ namespace MobileGL::MG_ConfigLoader {
         }
 
         // Everything the system files say, the backend file ahead of client.conf (first emplace
-        // wins). Re-read on every call: callers are few (Init, and a handful of lookups before it),
-        // and a file that changed between two processes' starts must not be served stale.
+        // wins). Re-read on every call: callers are few (Init, a handful of lookups before it, and a
+        // verify build's comparator when it arms), and a file that changed between two processes'
+        // starts must not be served stale.
         void ReadSystemConfig(UnorderedMap<String, String>& out, Bool log) {
             if (IsServerProcess()) return;
             ReadBackendFile(FilePathSetting("MOBILEGL_BACKEND_FILE", kDefaultBackendFile), out);

MobileGL/Init.h

@@ -28,7 +28,8 @@ namespace MobileGL {
         void Init();
         // One setting resolved the way Init() resolves it - the environment, then the system
         // configuration files (ConfigLoader.cpp) - for the few readers that must decide something
-        // before MobileGL initializes. False when neither names it.
+        // before MobileGL initializes, and for a verify build's comparator, which reads
+        // MOBILEGL_PIPE_VERIFY_CORRUPT when it arms. False when neither names it.
         Bool LookupSetting(const char* key, String& outValue);
     } // namespace MG_ConfigLoader
 

MobileGL/MG_Pipe/FatalFamilies.def

@@ -31,7 +31,7 @@
     X(UnnamedIdentity,               ProtocolCorruption, "a per-context memo was consulted with a null context identity") \
     X(PipeLegacyMemosDisabled,       ProtocolCorruption, "a subsystem bit is clear and this build compiles no legacy arm to fall back to (unreachable since P13 W3b fixed bits 0-13 on)") \
     X(PipeSubsystemsFixedOn,         ProtocolCorruption, "MOBILEGL_PIPE_PUSH cleared one of subsystem bits 0-13, which P13 fixed on") \
-    X(PipeVerifyBadKnob,             ProtocolCorruption, "a MOBILEGL_PIPE_VERIFY/POISON knob named a verb or field that does not exist") \
+    X(PipeVerifyBadKnob,             ProtocolCorruption, "MOBILEGL_PIPE_VERIFY_CORRUPT named a field that does not exist") \
     X(PipeVerifyDiffer,              ProtocolCorruption, "the verify comparator found a pushed field differing from the live one") \
     X(UnexpectedMapAccept,           ProtocolCorruption, "a persistent-map accept arrived where the protocol permits none") \
     X(UncarriedInitialBytes,         ProtocolCorruption, "a producer's initial bytes were required on the wire and absent") \