docs(changeset): record that predicates distribute over unions in v8

.changeset/predicates-distribute.md

@@ -0,0 +1,31 @@
+---
+'type-plus': major
+---
+
+The type predicates distribute over a union by default.
+
+A 7.x predicate checked a union as a whole, so a union with any member that failed answered `false`.
+A v8 predicate checks each member and combines the answers. A union whose members pass and fail now answers `boolean`:
+
+```ts
+type R = IsString<'a' | 1> // 7.x: false, 8.0: boolean
+type R = IsTrue<boolean> // 7.x: false, 8.0: boolean
+type R = IsNotString<'a' | 1> // 7.x: true,  8.0: boolean
+```
+
+The filter form keeps the members that pass. `IsString<'a' | 1, { selection: 'filter' }>` is `'a'`, where the 7.x `StringType<'a' | 1>` was `never`.
+
+This applies to `IsArray`, `IsBigint`, `IsBoolean`, `IsFalse`, `IsFunction`, `IsInteger`, `IsNull`, `IsNumber`, `IsNumeric`, `IsObject`, `IsString`, `IsSymbol`, `IsTrue`, `IsTuple`, `IsUndefined`, `IsVoid`, and the `IsNot*` negation of each.
+`IsPositive`, `IsNegative`, `IsNotPositive` and `IsNotNegative` already distributed in 7.x and do not change.
+
+A result tested with `extends true` behaves as before, because `boolean` does not extend `true`.
+A result tested with `extends false`, or compared with `IsEqual`, changes.
+
+Migration: pass `{ distributive: false }` to keep the 7.x result.
+
+```ts
+type R = IsString<'a' | 1, { distributive: false }> // false
+type R = IsString<'a' | 1, { distributive: false; selection: 'filter' }> // never
+```
+
+See [Predicates distribute over unions](https://cyberuni.github.io/type-plus/guides/migrating-to-v8/#predicates-distribute-over-unions) in the migration guide.