docs: clarify forked pull request token wording

[hiromieguchi802-lab]

Summary

• clarify that the write-token downgrade applies to forked pull request events except pull_request_target
• avoid implying that pull_request_target is subject to the same read-only adjustment

Fixes #32398.

Testing

• ran npm run lint-content -- --precommit --paths content/actions/reference/workflows-and-actions/workflow-syntax.md

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Table of review links

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The table shows the files in the content directory that were changed in this pull request. This helps you review your changes on a staging server. Changes to the data directory are not included in this table.

Source	Review	Production	What Changed
actions/reference/workflows-and-actions/workflow-syntax.md	fpt ghec ghes@ 3.20 3.19 3.18 3.17 3.16 3.15 3.14	fpt ghec ghes@ 3.20 3.19 3.18 3.17 3.16 3.15 3.14

Key: fpt: Free, Pro, Team; ghec: GitHub Enterprise Cloud; ghes: GitHub Enterprise Server

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