chore(book): add mention of E2EI state in database docs

Author: coriolinus

cc-book/modules/ROOT/pages/database.adoc

@@ -12,6 +12,7 @@ The database holds all state that must survive process restarts:
 * **Pending messages** — application messages and commits that arrived during an epoch transition and are waiting to be merged
 * **Proteus identity and sessions** — the device's long-lived Proteus identity keypair and the state of every active Proteus session
 * **Proteus prekeys** — including the last-resort prekey
+* **E2EI state** - data about the ACME certificate authority, various certificates and CRLs, refresh tokens, and the like
 
 All entries are encrypted at rest using the `DatabaseKey` provided at open time.