Add notes to stress call-during-initialisation

site/content/tutorial/15-context/01-context-api/text.md

@@ -54,3 +54,25 @@ In fact, you might use the two together. Since context is not reactive, values t
 ```js
 const { these, are, stores } = getContext(...);
 ```
+
+## `getContext()` must be called during component initialisation
+
+You may find this feature useful. However, remember to call `getContext()` during component initialisation! This means you can only use it directly in the `<script>` tag (for a `.svelte` file), not in any function. For example, this will work:
+
+```js
+import { getContext } from 'svelte';
+
+const map = getContext('map');
+```
+
+While this will not:
+
+```js
+import { getContext } from 'svelte';
+
+onMount(() => {
+  const map = getContext('map'); // will trigger an error!
+});
+```
+
+You may find it work in some old Svelte versions, but it won't work for current versions of Svelte.