Update wording to include prop behaviour

In regards to sveltejs#4442, this adds wording to explain that props are set to undefined when they are removed by the consumer.

site/content/docs/01-component-format.md

@@ -42,7 +42,7 @@ Svelte uses the `export` keyword to mark a variable declaration as a *property*
 
 ---
 
-You can specify a default value, which will be used if the component's consumer doesn't specify a prop.
+You can specify a default value for a prop. It will be used if the component's consumer doesn't specify the prop on the component. Note that whenever a specified prop is removed by the consumer, the value is set to `undefined` rather than the default value.
 
 In development mode (see the [compiler options](docs#svelte_compile)), a warning will be printed if no default is provided and the consumer does not specify a value. To squelch this warning, ensure that a default is specified, even if it is `undefined`.