Improve documentation for createParentChildLink

packages/gatsby/src/redux/actions.js

@@ -544,7 +544,7 @@ actions.createNodeField = (
 }
 
 /**
- * Creates a link between a parent and child node
+ * Creates a link between a parent and child node. This is used when you transform content from another node creating a new child node. You need to add the new child node to the `children` array of the parent but since you don't have direct access to the immutable parent node, use this action instead.
  * @param {Object} $0
  * @param {Object} $0.parent the parent node object
  * @param {Object} $0.child the child node object