docs(workers): clarify Version-Overrides header works on service-binding subrequests

[thomsoncf]

Summary

Adds a single paragraph + small JS snippet to the Version overrides section of the Gradual Deployments docs, clarifying that Cloudflare-Workers-Version-Overrides is honoured on subrequests made through a Worker-to-Worker service binding — but only when the calling Worker explicitly sets the header on the subrequest.

Why

The neighbouring Version affinity section already documents that Cloudflare-Workers-Version-Key works on service-binding subrequests:

You can set the Cloudflare-Workers-Version-Key header both when making an external request from the Internet to your Worker, as well as when making a subrequest from one Worker to another Worker using a service binding.

The same is true of Cloudflare-Workers-Version-Overrides, but the docs don't currently say so, which leads to repeat questions about whether the override header is honoured on service bindings.

There is also a subtle difference worth surfacing: the override header is not automatically propagated through a service binding — the calling Worker must explicitly include it on the subrequest. This PR makes that explicit with a short JS example using env.MY_SERVICE.fetch().

Verified empirically

Set up two Workers in one account:

• Callee with v1 (100%) and v2 (0%) in its gradual deployment.
• Caller with a service binding to the callee, conditionally setting the override header on env.CALLEE.fetch().

Scenario	Result
No override header	callee returns v1 (default)
Caller explicitly sets override → v2 on the subrequest	callee returns v2 ✅
Invalid version ID in override	falls back to v1 (matches existing docs)
Header set only on inbound request, caller does not forward it	callee returns v1 (no auto-propagation)

Out of scope

• RPC method calls (env.MY_SERVICE.someMethod()) — there is no API to attach headers per-call, so this PR doesn't address that case.
• Automatic propagation through service-binding chains — separate runtime work, not a docs change.