Clarify 'Web Origin' for third party original trail registration.

[tzuhsiao]

Fixes N/A

Changes proposed in this pull request:

This change clarifies 'Web Origin' for third party original trail registration. The current doc is a little bit confusing, it suggests that the token can be used for multiple third-party sites but it doesn't explain why we still need to provide 'Web Origin' in the registration. I found the answer in the troubleshooting page: the web origin should be the origin that serves the script.

[chrome-devrel-infra[bot]]

:white_check_mark: Preview (static) for commit 94dc433d7471f9d1071aece881ee3b3f87ffd0f6 available at https://pr-6568-static-dot-dcc-staging.uc.r.appspot.com/.

The following pages likely changed with this PR:

• https://pr-6568-static-dot-dcc-staging.uc.r.appspot.com/docs/web-platform/third-party-origin-trials/