Add MESOP_STATE_SESSION_BACKEND_SQL_CONNECTION_URI to config docs. (#824

)

docs/api/config.md

@@ -166,6 +166,13 @@ parameter specifies which Firestore collection that Mesop will write state sessi
 
 **Default:** `mesop_state_sessions`
 
+### MESOP_STATE_SESSION_BACKEND_SQL_CONNECTION_URI
+
+This is only used when the `MESOP_STATE_SESSION_BACKEND` is set to `sql`. This
+parameter specifies the database connection string. See the [SqlAlchemy docs for more details](https://docs.sqlalchemy.org/en/20/core/engines.html#database-urls).
+
+**Default:** `mesop_state_session`
+
 ### MESOP_STATE_SESSION_BACKEND_SQL_TABLE
 
 This is only used when the `MESOP_STATE_SESSION_BACKEND` is set to `sql`. This