2881 confusing site api related to meta and option (#2978)

* docs: update custom-fields documentation regarding ACF options fields

* update docs

docs/v2/guides/custom-fields.md

@@ -169,6 +169,9 @@ For site options, it’s also possible to access it directly through its name:
 
 Please be aware that using this might conflict with existing Timber methods on the `Timber\Site` object. That’s why the `option()` method is the preferred way to retrieve site options.
 
+
+You cannot fetch ACF options with `site.option()`. You will need to add the fields to the context yourself. This process is described in the [ACF integration](https://timber.github.io/docs/v2/integrations/advanced-custom-fields/#options-page) documentation.
+
 ## Query by custom field value
 
 This example that uses a [WP_Query](http://codex.wordpress.org/Class_Reference/WP_Query) array shows the arguments to find all posts where a custom field called `color` has a value of `red`.