doc: add externalBytes to heap profiler documentation

Document the externalBytes field in getAllocationProfile output.

Signed-off-by: Rudolf Meijering <[email protected]>

Author: rudolf

doc/api/v8.md

@@ -1503,12 +1503,17 @@ not running.
   "samples": [
     { "nodeId": 1, "size": 128, "count": 4, "sampleId": 42,
       "labels": { "route": "/users/:id" } }
+  ],
+  "externalBytes": [
+    { "labels": { "route": "/users/:id" }, "bytes": 1048576 }
   ]
 }
 ```
 
 * `samples[].labels` — key-value string pairs from the active label context
   at allocation time. Empty object if no labels were active.
+* `externalBytes[]` — live `Buffer`/`ArrayBuffer` backing-store bytes per
+  label context. Complements heap samples which only see the JS wrapper.
 
 ### `v8.withHeapProfileLabels(labels, fn)`
 
@@ -1551,9 +1556,9 @@ Prefer [`v8.withHeapProfileLabels()`][] when possible for automatic cleanup.
 ### Limitations — what is measured
 
 Heap samples cover V8 heap allocations (JS objects, strings, closures).
+`externalBytes` covers `Buffer`/`ArrayBuffer` backing stores.
 
-Not measured: `Buffer`/`ArrayBuffer` backing stores, native addon memory,
-JIT code space, OS-level allocations.
+Not measured: native addon memory, JIT code space, OS-level allocations.
 
 ## Class: `v8.GCProfiler`