Add documentation and release note for worker version overrides.

[jp4a50]

Note that the documentation for both version keys/affinity and version overrides are effectively "how to" guides. We may want to consider adding reference-style documentation for these headers.

Since there was no pre-existing header-based API, there is not an existing/obvious location in the docs to add this. We mention "Cloudflare headers" (i.e. headers which are set by Cloudflare) here which is the most appropriate place I can find. Interested to hear what others think is the best way to approach this.