fix(gatsby-source-filesystem) Clarify createFilePath docs (#26770)

* fix(gatsby-source-filesystem) Clarify createFilePath docs

Fixes: #26470

* Update README.md

Co-authored-by: Aisha Blake <[email protected]>

Author: keegan-lillo

packages/gatsby-source-filesystem/README.md

@@ -119,6 +119,7 @@ createFilePath({
   // The parameter from `onCreateNode` should be passed in here
   getNode,
   // The base path for your files.
+  // It is relative to the `options.path` setting in the `gatsby-source-filesystem` entries of your `gatsby-config.js`.
   // Defaults to `src/pages`. For the example above, you'd use `src/content`.
   basePath,
   // Whether you want your file paths to contain a trailing `/` slash