Note on the use of square brackets in GitHub Actions workflows filter patterns

[noritada]

Why:

Closes #18969

What's being changed (if available, include any code snippets, screenshots, or gifs):

This PR adds a following sentence and a code example:

Also, when you use [ and/or ] in a pattern within a flow sequence, the pattern must be enclosed in quotes.

# Valid
- [ master, 'release/v[0-9].[0-9]' ]

# Invalid - creates a parse error
- [ master, release/v[0-9].[0-9] ]

Check off the following:

• [x] I have reviewed my changes in staging (look for the "Automatically generated comment" and click the links in the "Preview" column to view your latest changes).
• [x] For content changes, I have completed the self-review checklist.

[welcome[bot]]

Thanks for opening this pull request! A GitHub docs team member should be by to give feedback soon. In the meantime, please check out the contributing guidelines.

[github-actions[bot]]

Automatically generated comment ℹ️

This comment is automatically generated and will be overwritten every time changes are committed to this branch.

The table contains an overview of files in the content directory that have been changed in this pull request. It's provided to make it easy to review your changes on the staging site. Please note that changes to the data directory will not show up in this table.

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Content directory changes

You may find it useful to copy this table into the pull request summary. There you can edit it to share links to important articles or changes and to give a high-level overview of how the changes in your pull request support the overall goals of the pull request.

Source	Preview	Production	What Changed
actions/using-workflows/workflow-syntax-for-github-actions.md	fpt ghec ghes@ 3.6 3.5 3.4 3.3 3.2 ghae	fpt ghec ghes@ 3.6 3.5 3.4 3.3 3.2 ghae

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fpt: Free, Pro, Team ghec: GitHub Enterprise Cloud ghes: GitHub Enterprise Server ghae: GitHub AE

[cmwilson21]

@noritada Thanks for opening a PR and linking it to your issue! 💛 I'll get this triaged for review! ⚡

[github-actions[bot]]

Thanks for opening a pull request! We've triaged this issue for technical review by a subject matter expert :eyes:

[github-actions[bot]]

This is a gentle bump for the docs team that this PR is waiting for technical review.

[github-actions[bot]]

This is a gentle bump for the docs team that this PR is waiting for technical review.

[github-actions[bot]]

Thanks very much for contributing! Your pull request has been merged 🎉 You should see your changes appear on the site in approximately 24 hours. If you're looking for your next contribution, check out our help wanted issues :zap: