docs: add glossary entry for Runner Group runs-on syntax (#54206)

Author: github-actions[bot]

docs/src/content/docs/reference/glossary.md

@@ -380,6 +380,10 @@ An extension mechanism for safe outputs that enables integration with third-part
 
 An experimental safe output type that triggers `repository_dispatch` events in external repositories for cross-repository orchestration. Each key under `safe-outputs.dispatch-repository:` defines a named tool exposed to the agent. A tool requires a `workflow` identifier (forwarded in `client_payload` for routing), an `event_type`, and either a static `repository` slug or an `allowed_repositories` list. GitHub Actions expressions (`${{ ... }}`) are supported in repository fields and are passed through without format validation. At compile time the compiler emits a warning: `Using experimental feature: dispatch-repository`. See [Safe Outputs Reference](/gh-aw/reference/safe-outputs/#repository-dispatch-dispatch-repository).
 
+### Runner Group (`runs-on: {group: ...}`)
+
+An object form of the `runs-on:` field that targets a named GitHub Actions runner group, optionally filtered by `labels:`. Supported by the top-level `runs-on`, `runs-on-slim`, `safe-outputs.runs-on`, `safe-outputs.threat-detection.runs-on`, and custom `safe-outputs.jobs.<job>.runs-on` fields, alongside the plain string and label-array forms. Useful for routing custom safe-jobs or framework jobs to a specific pool of self-hosted runners. See [Self-Hosted Runners Reference](/gh-aw/reference/self-hosted-runners/#runs-on-formats).
+
 ### Safe Output Actions
 
 A mechanism for mounting any public GitHub Action as a once-callable MCP tool within the consolidated safe-outputs job. Defined under `safe-outputs.actions:`, each action is specified with a `uses` field (matching GitHub Actions syntax) and an optional `description` override. At compile time, `gh aw compile` fetches the action's `action.yml` to resolve its inputs and pins the reference to a specific SHA. Unlike [Custom Safe Outputs](#custom-safe-outputs) (separate jobs) and [Safe Output Scripts](#safe-output-scripts) (inline JavaScript), actions run as steps inside the safe-outputs job with full secret access via `env:`. Useful for reusing existing marketplace actions as agent tools. See [Custom Safe Outputs](/gh-aw/reference/custom-safe-outputs/#github-action-wrappers-safe-outputsactions).