document reserved words in React context (#12424)

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## Description

Documenting limitations with React context, as per the feedback/comments in #11787.

Closes #12423

docs/docs/creating-and-modifying-pages.md

@@ -39,8 +39,12 @@ your sites development server at `HOST:PORT/___graphql` e.g.
 }
 ```
 
+The `context` property accepts an object, and we can pass in any data we want the page to be able to access.
+
 You can also query for any `context` data you or plugins added to pages.
 
+> **NOTE:** There are a few reserved names that _cannot_ be used in `context`. They are: `path`, `matchPath`, `component`, `componentChunkName`, `pluginCreator___NODE`, and `pluginCreatorId`.
+
 ## Creating pages in gatsby-node.js
 
 Often you will need to programmatically create pages. For example, you have