fixes app block validation per: panoply/vscode-liquid#175

CC David

stores/descriptions/sections/properties/@app.md

@@ -0,0 +1,46 @@
+App block wrapperAnchor link to section titled **"App block wrapper"**. Merchants can add app blocks to a page in the following ways:
+
+- As a block within the confines of the section that's rendering the block
+- In a similar manner to sections, giving them the full width of the page to render content
+
+As app blocks aren't sections themselves, Shopify wraps these full-width app blocks in a platform-generated section by default. However, you can override this default section by creating your own section called apps.liquid.
+
+The `apps.liquid` section schema needs to include a block of type `@app`, as well as a `preset`. If either of these is missing, then an `Apps not supported` or `Apps section is invalid` error is returned in the theme editor and merchants aren't able to use the section.
+
+> **CAUTION**
+>
+> The `apps.liquid` section schema can't contain the `templates` schema attribute. This also includes the `templates` attribute within the `enabled_on/disabled_on` schema attributes. It's expected that the `apps.liquid` section is available on all templates.
+
+**Example**
+
+```liquid
+
+{% for block in section.blocks %}
+  {% render block %}
+{% endfor %}
+
+{% schema %}
+  {
+    "name": "App wrapper",
+    "settings": [],
+    "blocks": [
+      {
+        "type": "@app"
+      }
+    ],
+    "presets": [
+      {
+        "name": "App wrapper"
+      }
+    ]
+  }
+{% endschema %}
+
+
+```
+
+#
+
+---
+
+[Shopify Documentation](https://shopify.dev/docs/storefronts/themes/architecture/blocks/app-blocks#app-block-wrapper)