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-core/src/settings.rs

@@ -40,6 +40,8 @@
 //!      `lines`) are the one text the full-TIN check doesn't scan, because
 //!      a routing number is nine digits (A-3); every other text in the
 //!      list is scanned.
+//!   8. An invoice number's prefix is 1 to 12 visible ASCII characters:
+//!      longer, blank or holding a space would not read as one number.
 //! ```
 
 use crate::TaxForm;