docs: update pr-tags readme (#6734)

### Summary

Clarifying the wording around when new test tags get picked up by test
runner.

### QA Notes

n/a

Author: midleman

test/e2e/README.md

@@ -194,23 +194,28 @@ For R, add any package requirements to the "imports" section of the `DESCRIPTION
 
 ## Pull Requests and Test Tags
 
-When you create a pull request, the test runner automatically scans the PR description for test tags to determine which E2E tests to run.
+### Test Tag Rules
+
+When creating a pull request, the test runner automatically scans the PR description for test tags to determine which E2E tests to run.
 
 - **Always-on Tests:** Tests tagged with `@:critical` always run, and you can’t opt out of them.
 - **Custom Tags:** If your changes affect a specific feature, you can include additional tags in the PR description to trigger relevant tests.
 
-To add a test tag:
+### How to Add a Test Tag
 
-1. Use the format `@:tag` in your PR description (e.g., `@:help`, `@:console`).
+1. Use the format `@:tag` anywhere in your PR description (e.g., `@:help`, `@:console`).
 2. Once added, a comment will appear on your PR confirming that the tag was found and parsed correctly.
 
-From that point, all E2E tests linked to the specified tag(s) will run during the test job. For a full list of available tags, see this [file](https://github.com/posit-dev/positron/blob/main/test/e2e/infra/test-runner/test-tags.ts).
+> [!NOTE]
+> **Add tags before the `pr-tags` job starts**. If you update tags _after_ opening the PR, push a new commit or restart the jobs to apply the changes. The PR comment will confirm the detected tags, and tests will run based on the tags present at execution time.
+For a full list of available tags, see this [file](https://github.com/posit-dev/positron/blob/main/test/e2e/infra/test-runner/test-tags.ts).
 
-To include Windows and Web Browser testing:
+### Running Windows and Browser Tests
 
-By default, only Linux e2e test will run.  You can optionally add `@:win` to your PR description and this will run test on windows as well. As of now, windows tests do take longer to run, so the overall PR test job will take longer to complete. You can also ass `@:web` to run the browser tests.
+By default, only Linux E2E tests run. If you need to include additional environments:
 
-Note: You can update the tags in the PR description at any time. The PR comment will confirm the parsed tags, and the test job will use the tags present in the PR description at the time of execution.
+- Add `@:win` to your PR description to run tests on Windows. (Note: Windows tests take longer to complete.)
+- Add `@:web` to run browser-based tests.
 
 ## Running Tests in Github Actions