doc: note the system requirements for V8 tests

The `test-v8` Makefile target still requires Python 2,
and it requires a full Xcode install on macOS.

Refs: nodejs#36691

doc/guides/maintaining-V8.md

@@ -229,7 +229,7 @@ to be cherry-picked in the Node.js repository and V8-CI must test the change.
   * Run the Node.js [V8 CI][] in addition to the [Node.js CI][].
     The CI uses the `test-v8` target in the `Makefile`, which uses
     `tools/make-v8.sh` to reconstruct a git tree in the `deps/v8` directory to
-    run V8 tests.
+    run V8 tests<sup>2</sup>.
 
 The [`git-node`][] tool can be used to simplify this task. Run
 `git node v8 backport <sha>` to cherry-pick a commit.
@@ -414,6 +414,11 @@ This would require some tooling to:
 <sup>1</sup>Node.js 0.12 and older are intentionally omitted from this document
 as their support has ended.
 
+<sup>2</sup>At the moment the V8 tests still require Python 2. To run these
+tests locally, you can run `/path/to/python2 ./configure.py` before running
+`make test-v8`, in the root of this repository. On macOS, this also requires
+a full Xcode install, not just the "command line tools" for Xcode.
+
 [ChromiumReleaseCalendar]: https://www.chromium.org/developers/calendar
 [Node.js CI]: https://ci.nodejs.org/job/node-test-pull-request/
 [Node.js `canary` branch]: https://github.com/nodejs/node-v8/tree/canary