Clarify that vars context is unavailable in composite actions

[imaloveabra]

Why:

Closes: #46232

The vars context is currently documented without mentioning that it's unavailable inside composite actions. This causes confusion as users expect ${{ vars.MY_VAR }} to work.

What's being changed:

Added a NOTE clarifying that vars is not available in composite actions and showing the workaround to pass it as an input from the calling workflow.

Check off the following:

• I have reviewed the docs fundamentals
• All CI checks are passing and the changes look good

[github-actions]

Please don't raise pull requests against an existing issue that another user has raised until it's been marked with a help wanted label. We need to properly assess all issues before we can accept community contributions, and determine whether it's something we need to fix internally, or how many articles are affected and what the scope of the fix will be. For now, we're going to close this pull request.

[github-actions]

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[github-actions]

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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/contexts.md	fpt ghec ghes@ 3.22 3.21 3.20 3.19 3.18	fpt ghec ghes@ 3.22 3.21 3.20 3.19 3.18

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

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