Update machine restart policy docs

Add extra details to `on-fail` restart policy documentation:
- clarify that the `max_restart` value is configurable, with a default 10
- add detail that the max number of restarts is evaluated within a fixed 5-minute window

machines/guides-examples/machine-restart-policy.html.markerb

@@ -13,7 +13,7 @@ The restart policies are:
 
 - **`always`**: Always restart a Machine automatically and never let it enter a `stopped` state, even when the main process exits cleanly. `always` is the default when you create a Machine with `fly m run` and for Fly Postgres app Machines. Recommended for "always-on" apps with no services configured, since the Machine restarts regardless of the exit code.
 
-- **`on-fail`** (or **`on-failure`** for the Machines API and when viewed in the Machine config): Try up to 10 times to automatically restart the Machine if it exits with a non-zero exit code, before letting it stop. Recommended for most Machines with services configured, since Fly Proxy can wake them on request. `on-fail` lets Machines be restarted if they crash, and allows your app Machines to effectively scale down by exiting cleanly. `on-fail` is the default when there's no explicit restart policy in a Machine's config, such as Machines created by `fly launch` and `fly deploy`. Machines with a schedule also default to the `on-fail` restart policy.
+- **`on-fail`** (or **`on-failure`** for the Machines API and when viewed in the Machine config): Try up to `max_restart` (default 10) times within a 5-minute window to automatically restart the Machine if it exits with a non-zero exit code, before letting it stop. Recommended for most Machines with services configured, since Fly Proxy can wake them on request. `on-fail` lets Machines be restarted if they crash, and allows your app Machines to effectively scale down by exiting cleanly. `on-fail` is the default when there's no explicit restart policy in a Machine's config, such as Machines created by `fly launch` and `fly deploy`. Machines with a schedule also default to the `on-fail` restart policy.
 
 ## Check a Machine's restart policy