Document email header fallback for provision endpoint - #37

Running Vale across on the following files affected by this PR:

- api-reference/agents.mdx

Attempting to initialize Vale linter...
No customer vale configuration detected. Using Mintlify's default configuration.
Attempting to run Vale spellcheck...
Vale spellcheck completed successfully.

api-reference/agents.mdx:
	- 478:66    Did you really mean 'Ethereum'?
	- 551:16    Did you really mean 'proxied'?
	- 551:73    Did you really mean 'Mux'?
	- 588:59    Did you really mean 'userId'?
	- 588:89    Did you really mean 'streamKey'?
	- 842:87    Did you really mean 'unconfigured'?


Found 6 suggested changes across 1 files in 826.29ms