refactor(settings): the slice's contracts and notes (P4b S4.2)

invoicing.first_number's refusal (LedgerError::FirstNumberLocked) lands
as the contract the store's check raises once invoices carry numbers
(S4.3): a series that has begun is never renumbered (B-40, A-2).

Also: the settings invariants name the number prefix's bound, update_party's
doc says which facts wait for documents_v2, and the email check no longer
reads as a single-arm match. Terms is S4.1's as it landed; this slice no
longer carries a copy.

crates/ledger-store/src/store.rs

@@ -1597,6 +1597,10 @@ impl Store {
     }
 
     /// Change one of a party's facts. Journaled as `party.updated`.
+    ///
+    /// An email or a default terms fact needs the `documents_v2` feature
+    /// (plan P4b S4.2): books that haven't declared it refuse the write
+    /// with the upgrade remedy, journaling nothing.
     pub fn update_party(
         &mut self,
         id: ledger_core::PartyId,