Add available contexts and special functions at snapshot condition

Author: rhysd

content/actions/reference/workflows-and-actions/contexts.md

@@ -102,6 +102,7 @@ The following table lists the restrictions on where each context and special fun
 | `jobs.<job_id>.services` | `github, needs, strategy, matrix, vars, inputs` | None |
 | `jobs.<job_id>.services.<service_id>.credentials` | `github, needs, strategy, matrix, env, vars, secrets, inputs` | None |
 | `jobs.<job_id>.services.<service_id>.env.<env_id>` | `github, needs, strategy, matrix, job, runner, env, vars, secrets, inputs` | None |
+| `jobs.<job_id>.snapshot.if` | `github, needs, strategy, matrix, vars, inputs` | None |
 | `jobs.<job_id>.steps.continue-on-error` | `github, needs, strategy, matrix, job, runner, env, vars, secrets, steps, inputs` | `hashFiles` |
 | `jobs.<job_id>.steps.env` | `github, needs, strategy, matrix, job, runner, env, vars, secrets, steps, inputs` | `hashFiles` |
 | `jobs.<job_id>.steps.if` | `github, needs, strategy, matrix, job, runner, env, vars, steps, inputs` | `always, cancelled, success, failure, hashFiles` |