Clarify available contexts and special functions at snapshot condition

[rhysd]

Why:

Closes: #41255

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

Added job.<job_id>.snapshot.if to the available contexts table based on the runner implementation as follows:

• Available contexts: https://github.com/actions/runner/blob/c96dcd472907e514274cdb4af281116adf7aad18/src/Sdk/WorkflowParser/Conversion/WorkflowTemplateConverter.cs#L2216
• Available special functions: https://github.com/actions/runner/blob/c96dcd472907e514274cdb4af281116adf7aad18/src/Sdk/WorkflowParser/Conversion/WorkflowTemplateConverter.cs#L2240

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]

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Source	Review	Production	What Changed
actions/reference/workflows-and-actions/contexts.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

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[Copilot]

Pull request overview

This PR adds documentation for the jobs.<job_id>.snapshot.if conditional to the contexts and special functions table. The addition clarifies which contexts (github, needs, strategy, matrix, vars, inputs) and special functions (None) are available when using the if condition with the snapshot job configuration, based on the GitHub Actions runner implementation.

Key changes:

• Added a single row to the contexts availability table documenting jobs.<job_id>.snapshot.if

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[rhysd]

CI seemed to fail for the reason unrelated to the changes in this PR.