docs: clarify matrix context property names are case-sensitive

[sushant-me]

Closes #27200. Adds a note to the \matrix\ context section clarifying that matrix property names are case-sensitive (e.g. \matrix.arch\ vs \matrix.Arch).

[github-actions]

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actions/reference/workflows-and-actions/contexts.md	fpt ghec ghes@ 3.22 3.21 3.20 3.19 3.18 3.17	fpt ghec ghes@ 3.22 3.21 3.20 3.19 3.18 3.17

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[subatoi]

Thanks for submitting a PR, @sushant-me — we'll consider this in the context of the original issue, but since that wasn't approved for user contributions I'm unable to review this at this time. I'm happy to leave this open, but I'd also note that the original contributor may prefer to raise their own PR in the event that a contribution's approved. Thank you