Add explanation to `CI` default env variable

[Marcono1234]

Why:

Currently the purpose of the CI variable might not be immediately obvious.

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

Simply copied the description from the GITHUB_ACTIONS variable, which serves the same purpose.

Check off the following:

• [ ] A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• [ ] The changes in this PR meet the docs fundamentals that are required for all content.
• [ ] All CI checks are passing and the changes look good in the review environment.

[github-actions[bot]]

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Table of review links

Note: Please update the URL for your staging server or codespace.

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/variables.md	fpt ghec ghes@ 3.19 3.18 3.17 3.16 3.15 3.14	fpt ghec ghes@ 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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[Sharra-writes]

@Marcono1234 Thanks for opening a PR! I will check in and see if this is something the Actions team wants to add.