docs: update README instructions for updating WPT (#32124)

I noticed in #32123 that there are two issues with the docs for updating
Web Platform Tests:

- `./mach update-wpt --sync` fails with an error (`Are you sure you
don't want a patch?`) because it expects a `--patch` arg.
- The `tests/wpt/meta-legacy-layout/` test results need to be updated
with a separate command.

This updates `tests/wpt/README.md` with more complete instructions.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by
`[X]` when the step is complete, and replace `___` with appropriate
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #___ (GitHub issue number if applicable)

<!-- Either: -->
- [ ] There are tests for these changes OR
- [x] These changes do not require tests because this is a docs-only
change

<!-- Also, please make sure that "Allow edits from maintainers" checkbox
is checked, so that we can help you if you get stuck somewhere along the
way.-->

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tests/wpt/README.md

@@ -162,11 +162,22 @@ remove `.ini` files that no longer contain any expectations.
 
 When a larger number of changes is required, this process can be automated.
 This first requires saving the raw, unformatted log from a test run, for
-example by running `./mach test-wpt --log-raw /tmp/servo.log`. Once the
-log is saved, run from the root directory:
+example by running:
+
+    ./mach test-wpt --log-raw /tmp/servo.log
+
+Once the log is saved, run from the root directory:
 
     ./mach update-wpt /tmp/servo.log
 
+The same process can be done for the legacy layout engine:
+
+    ./mach test-wpt --legacy-layout --log-raw /tmp/servo-legacy.log
+    ./mach update-wpt --legacy-layout /tmp/servo-legacy.log
+
+The updated expectations will be in `tests/wpt/meta/` and
+`tests/wpt/meta-legacy-layout/` respectively.
+
 Writing new tests
 =================
 
@@ -196,19 +207,6 @@ web-platform-tests may be edited in-place and the changes committed to
 the servo tree. These changes will be upstreamed when the tests are
 next synced.
 
-Updating the upstream tests
-===========================
-
-In order to update the tests from upstream use the same mach update
-commands. e.g. to update the web-platform-tests:
-
-    ./mach update-wpt --sync
-    ./mach test-wpt --log-raw=update.log
-    ./mach update-wpt update.log
-
-This should create two commits in your servo repository with the
-updated tests and updated metadata.
-
 Servo-specific tests
 ====================