Clarify that not all blocks support nested content

docs/block-api.md

@@ -268,7 +268,7 @@ useOnce: true,
 
 * **Type:** `Array`
 
-Blocks can be inserted into other blocks as nested content. Sometimes it is useful to restrict a block so that it is only available as a nested block. For example, you might want to allow an 'Add to Cart' block to only be available within a 'Product' block.
+Blocks are able to be inserted into blocks that use [`InnerBlocks`](https://github.com/WordPress/gutenberg/blob/master/editor/components/inner-blocks/README.md) as nested content. Sometimes it is useful to restrict a block so that it is only available as a nested block. For example, you might want to allow an 'Add to Cart' block to only be available within a 'Product' block.
 
 Setting `parent` lets a block require that it is only available when nested within the specified blocks.