Merge: Improve documentation (#1375)

Author: sindresorhus

source/merge.d.ts

@@ -12,6 +12,31 @@ type SimpleMerge<Destination, Source> = Simplify<{
 /**
 Merge two types into a new type. Keys of the second type overrides keys of the first type.
 
+This is different from the TypeScript `&` (intersection) operator. With `&`, conflicting property types are intersected, which often results in `never`. For example, `{a: string} & {a: number}` makes `a` become `string & number`, which resolves to `never`. With `Merge`, the second type's keys cleanly override the first, so `Merge<{a: string}, {a: number}>` gives `{a: number}` as expected. `Merge` also produces a flattened type (via `Simplify`), making it more readable in IDE tooltips compared to `A & B`.
+
+@example
+```
+import type {Merge} from 'type-fest';
+
+type Foo = {
+	a: string;
+	b: number;
+};
+
+type Bar = {
+	a: number; // Conflicts with Foo['a']
+	c: boolean;
+};
+
+// With `&`, `a` becomes `string & number` which is `never`. Not what you want.
+type WithIntersection = (Foo & Bar)['a'];
+//=> never
+
+// With `Merge`, `a` is cleanly overridden to `number`.
+type WithMerge = Merge<Foo, Bar>['a'];
+//=> number
+```
+
 @example
 ```
 import type {Merge} from 'type-fest';