Answer product-owner round 1: name the field exchange, and say in plain words what phone-exchange draws

CHANGELOG.md

@@ -90,8 +90,9 @@ replacement, and each removed path, column or flag.
   and carries the same three. `person.prefix` is null where a person has no title,
   where it used to be an empty string, so `--format sql` writes `NULL` and a `string`
   struct field reading it becomes `*string`.
-- `en_US.phone-exchange` draws a central office code NANPA can assign: `200` to `999`,
-  never `N11`, `555`, `950`, `958`, `959` or `976`. `en_US.phone` draws its `exch` from it.
+- `en_US.phone-exchange` draws the three digits after a US area code, as NANPA assigns
+  them: `200` to `999`, never `211` to `911` ending in `11`, nor `555`, `950`, `958`,
+  `959` or `976`. `en_US.phone` draws its `exchange` from it.
 - `ErrNoColumns` is exported, so a caller can tell the one record fence a path can
   answer from the rest.
 - An error names a spelling that runs: a layout is named single-quoted and free of