Merge pull request #4502 from alphagov/document_heading_leve

Document heading_level for heading blocks

docs/building_blocks_for_flexible_content.md

@@ -115,12 +115,13 @@ When added through Whitehall content can either be written in HTML or govspeak m
 
 A wrapper around the [Heading component](https://components.publishing.service.gov.uk/component-guide/heading). This block uses `content` where the component uses `text` for the title text key. This is so that headings can appear in search (only values inside a `content` key will be indexed when being published from Whitehall - see [Indexing block content in search](#indexing-block-content-in-search))
 
-If an `id` is added, the heading can be referenced in an anchor link.
+If an `id` is added, the heading can be referenced in an anchor link. `heading_level` can be used to specify the heading level (the default is 2) - you should usually have at least one `heading_level: 1` towards the beginning of a page's content.
 
 ```yaml
 - type: heading
   id: heading-id
   content: Porem ipsum dolor
+  heading_level: 3
 ```
 
 #### Image