docs: add a troubleshooting note for hover tests with headed parallel browsers

[goosewobbler]

Proposed changes

Adds a Troubleshooting entry to platforms/Web.md for hover and moveTo() flakes with headed parallel browsers. Another worker's window opening or closing under the OS pointer sends real mouse events to the page. webdriverio#15960 hit this on the Windows runner, and the same happens under Xvfb on Linux, where v10 runs all workers on one display.

How you tested

Reproduced it in Docker: under Xvfb, a hover test failed when another worker's window opened or closed over the pointer. It passed with the pointer moved away, and under Weston.

• Unit tests for the touched package(s)
• Type definition tests (if the public API / command types changed)
• Smoke or e2e (only if the testrunner, launcher, or a real browser path changed)

Types of changes

• Polish (an improvement for an existing feature)
• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update (improvements to the project's docs)
• Specification changes (updates to WebDriver command specifications)
• Internal updates (everything related to internal scripts, governance documentation and CI files)

Checklist

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have added the necessary documentation (if appropriate)
• I have added proper type definitions for new commands (if appropriate)

Backport Request

• This change is solely for v10 and doesn't need to be back-ported
• Back-ported PR at #XXXXX

[goosewobbler]

Raised upstream as webdriverio#16002.