explain why type argument was removed

Author: Rich-Harris

sites/svelte-5-preview/src/routes/docs/content/01-api/02-runes.md

@@ -473,6 +473,15 @@ type MyProps = any;
 let { a, b, c, ...everythingElse }: MyProps = $props();
 ```
 
+> In an earlier preview, `$props()` took a type argument. This caused bugs, since in a case like this...
+>
+> ```ts
+> // @errors: 2558
+> let { x = 42 } = $props<{ x: string }>();
+> ```
+>
+> ...TypeScript [widens the type](https://www.typescriptlang.org/play?#code/CYUwxgNghgTiAEAzArgOzAFwJYHtXwBIAHGHIgZwB4AVeAXnilQE8A+ACgEoAueagbgBQgiCAzwA3vAAe9eABYATPAC+c4qQqUp03uQwwsqAOaqOnIfCsB6a-AB6AfiA) of `x` to be `string | number`, instead of erroring.
+
 Props cannot be mutated, unless the parent component uses `bind:`. During development, attempts to mutate props will result in an error.
 
 ### What this replaces