docs: add section on using expressions in credentials

docs/data/expressions-for-transformation.md

@@ -58,6 +58,18 @@ This expression:
 1. Accesses the incoming JSON-formatted data using n8n's custom `$json` variable.
 2. Finds the value of `city` (in this example, "New York"). Note that this example uses JMESPath syntax to query the JSON data. You can also write this expression as `{{$json['body']['city']}}`.
 
+### Using expressions in credentials
+
+You can also use expressions in credential fields. When you reference data using expressions (for example, `{{$json.body.city}}` or `{{ $('Webhook').item.json.headers.authorization }}`), n8n evaluates the expression within the context of the current workflow execution.
+
+This means that:
+
+- Expressions in credentials can access data available in the current execution context, including data from previous nodes.
+- Each workflow execution has its own data context.
+- Expressions are evaluated per execution, so different executions do not share data.
+
+For example, if a webhook node receives an access token and you reference it in a credential field using an expression, the value is resolved using the execution data of that specific workflow run.
+
 ## Example: Writing longer JavaScript as expressions
 
 You can do things like variable assignments or multiple statements in an expression, but you need to wrap your code using the syntax for an Immediately Invoked Function Expression (IIFE).