Additions to the protocol docs

[csymlstd]

While working on a JS adapter, I found a few pieces missing from the protocol docs that might help others.

Error bags will send the value through the X-Inertia-Error-Bag header, which the adapter uses to namespace the messages on the shared errors prop.

I have run into some caching issues in production as the Accept request header uses text/html. The Laravel adapter modifies the Vary header for every request to include X-Inertia. While not necessarily a requirement for the adapter to work, it is default behavior that can fix caching in most scenarios* in all adapters. The Inertia responses section should maybe display a different response:

HTTP/1.1 200 OK
Content-Type: application/json
Vary: X-Inertia, Accept
X-Inertia: true

*this unfortunately does not work with Cloudflare caching, as they do not use the Vary header in the cache key unless you are on an enterprise plan that can customize keys.