docs: add some missing details around string coercion and handling of nullish values (#15739)

closes #14716

Author: Rich-Harris

documentation/docs/02-runes/05-$props.md

@@ -37,7 +37,7 @@ On the other side, inside `MyComponent.svelte`, we can receive props with the `$
 
 ## Fallback values
 
-Destructuring allows us to declare fallback values, which are used if the parent component does not set a given prop:
+Destructuring allows us to declare fallback values, which are used if the parent component does not set a given prop (or the value is `undefined`):
 
 ```js
 let { adjective = 'happy' } = $props();
@@ -219,4 +219,4 @@ This is useful for linking elements via attributes like `for` and `aria-labelled
 	<label for="{uid}-lastname">Last Name: </label>
 	<input id="{uid}-lastname" type="text" />
 </form>
-```
+```

documentation/docs/03-template-syntax/01-basic-markup.md

@@ -154,6 +154,8 @@ A JavaScript expression can be included as text by surrounding it with curly bra
 {expression}
 ```
 
+Expressions that are `null` or `undefined` will be omitted; all others are [coerced to strings](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String#string_coercion).
+
 Curly braces can be included in a Svelte template by using their [HTML entity](https://developer.mozilla.org/docs/Glossary/Entity) strings: `{`, `{`, or `{` for `{` and `}`, `}`, or `}` for `}`.
 
 If you're using a regular expression (`RegExp`) [literal notation](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp#literal_notation_and_constructor), you'll need to wrap it in parentheses.