Follow-ups for redirection support

[eddyashton]

#6050 provides a working implementation of forwarding, but there's a few things that I've delayed for future PRs:

• Removing the custom is_create flag. I think the special /create endpoint can be given appropriate flags so that it is always executed locally, and doesn't need a custom flag. Remove is_create_request flag #6096
• Revisit fallback to local execution. This was added to the redirection path for parity with the forwarding path. I believe hope it can be removed, but we may currently depend on it during unit tests or early-lifecycle execution. Move unnecessary compatibility path #6097
• Clarify interaction of set_forwarding_required and set_redirection_strategy. We currently derive the latter from the former, need to make sure we do that safely, as we move towards deprecation of the former.
• Improve documentation. The auto-generated host schema docs don't give much detail on the new fields, need to expand what they show. Update RST docs generation from JSON schema #6095
• Add to_backup forwarding option. No explicit demand for this, but should be easy to add for balance.
• Add sample of JS app configuration.