chore(test runner): document that --only-changed on CI needs history

Closes https://github.com/microsoft/playwright/issues/32452

--only-changed needs more than a single commit locally to work properly. `fetch-depth: 0` does that, see https://github.com/actions/checkout?tab=readme-ov-file#fetch-all-history-for-all-tags-and-branches.

Signed-off-by: Simon Knott <info@simonknott.de>

docs/src/ci.md

@@ -432,6 +432,8 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v4
+      with:
+        fetch-depth: 0
     - uses: actions/setup-node@v4
       with:
         node-version: 18
@@ -1044,4 +1046,4 @@ xvfb-run mvn test
 ```
 ```bash csharp
 xvfb-run dotnet test
-```
+```