Add docs for fly-replay-cache-control and fly-replay-cache-status (#2216)

* Add docs for `fly-replay-cache-control` and `fly-replay-cache-status`

* Apply suggestions from code review

Co-authored-by: Kristin Martin <[email protected]>

* More suggestions

---------

Co-authored-by: Kristin Martin <[email protected]>

Author: PeterCxy

networking/dynamic-request-routing.html.markerb

@@ -168,6 +168,39 @@ The app issuing `fly-replay` still serves as the ultimate source of truth, and w
 To ensure reliable operation, the app issuing `fly-replay` should still have multiple instances deployed in multiple regions.
 </div>
 
+### Bypassing the replay cache
+
+Sometimes, you might want a client to bypass a cached replay without actually invalidating the cache.
+For example, you might have an API endpoint that _mostly_ needs to be replayed to a "leader" instance. But depending on application logic, it could sometimes be handled by any instance.
+
+The recommended approach is to clearly separate endpoints that require replay from those that don't.
+However, when that is not possible, the replaying app can additionally set the header
+
+```
+fly-replay-cache-allow-bypass: yes
+```
+
+or set the JSON field `"allow_bypass": true`. Then, when your client makes a request, it can set the header:
+
+```
+fly-replay-cache-control: skip
+```
+
+to skip any cached replay that might be present.
+
+<div class="note icon">
+**Note**: This behavior is opt-in because enabling it by default could make your app vulnerable to malicious clients creating unexpected load on the replaying machine. All it takes is setting a boolean HTTP header.
+</div>
+
+### Was my request served by the cache?
+
+If your app needs to know whether a request was served from the replay cache, it can check the fly-replay-cache-status header. This header is sent to the replay _target_ app or machine.
+
+- Absent: the request was not replayed and was delivered directly to the app or machine.
+- `miss`: the request didn't use replay cache; it first hit another machine and got replayed here.
+- `hit`: the request did use the replay cache! It never reached the source app or machine and was redirected straight to the target.
+- `bypass`: even though the cache might have been set, it was explicitly skipped by setting the `fly-replay-cache-control: skip` header.
+
 ## Implementation Details
 
 ### Requirements and Limitations