docs: add backend limitation (#31607)

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Co-authored-by: sebhcp <sebhcp@users.noreply.github.com>
Co-authored-by: Laura Pacilio <83350965+laurapacilio@users.noreply.github.com>

website/docs/language/settings/backends/configuration.mdx

@@ -46,6 +46,7 @@ There are some important limitations on backend configuration:
 
 - A configuration can only provide one backend block.
 - A backend block cannot refer to named values (like input variables, locals, or data source attributes).
+- You cannot reference values declared within backend blocks elsewhere in the configuration. Refer to [References to Resource Attributes](/terraform/language/expressions/references#references-to-resource-attributes) for more details.
 
 ### Credentials and Sensitive Data