Say what the objectid category actually is, and re-flow the registry comment

README.md

@@ -178,9 +178,9 @@ Swedish personnummer): `address`, `color`, `company`, `date`, `email`, `ip`,
 `data/misc` carries locale-neutral categories — universal data that isn't tied to
 a language or region:
 
-- ids & networking: `uuid` (a proper random v4), `mac`, `objectid` (MongoDB
+- ids & networking: `uuid` (a proper random v4), `mac`, `objectid` (24 hex chars,
-  ObjectID, 24 hex chars), `creditcard` (per-network numbers ending in a valid
+  MongoDB ObjectID-shaped — the leading bytes are random, not a real timestamp),
-  `{luhn()}` digit)
+  `creditcard` (per-network numbers ending in a valid `{luhn()}` digit)
 - reference codes: `currency` (ISO 4217), `country` (ISO 3166), `language` (ISO
   639), `timezone` (IANA)
 - web & systems: `mimetype`, `httpstatus`, `useragent`

builtins.go

@@ -20,9 +20,9 @@ const (
 // the digits emitted so far in the current expansion (luhn, mod11, ean — place
 // them after their payload); generators read only the rng (uuid, ulid, ...). All
 // must stay pure over (rng, emitted, args) so a seeded faker is reproducible — a
-// time-based id (uuid v7, ulid) draws its timestamp from the rng, not
+// time-based id (uuid v7, ulid) draws its timestamp from the rng, not the wall
-// the wall clock. Add a builtin only for what data can't express: a random v4
+// clock. Add a builtin only for what data can't express: a random v4 UUID and a
-// UUID already ships as data (data/misc/uuid.json), so the builtin is v7.
+// 24-hex ObjectID both ship as data, so the uuid builtin is v7.
 var builtins = map[string]builtin{
 	"luhn": {arity: 0, call: func(_ *session, e string, _ map[string]node, _ []string) string {
 		return string(rune('0' + luhnCheck(e)))