Update docs for report mixin (#475)

docs/extend/plugins/report.md

@@ -6,6 +6,28 @@ title: Report Mixin
 
 The ReportMixin class provides a plugin with the ability to extend the functionality of custom [report templates](../../report/report.md). A plugin which implements the ReportMixin mixin class can add custom context data to a report template for rendering.
 
+### Add Report Context
+
+Custom context data can be provided to a report template using the `add_report_context` method:
+
+```python
+def add_report_context(self, report_instance, model_instance, request, context):
+    """Add extra context to the provided report instance.
+
+    By default, this method does nothing.
+
+    Args:
+        report_instance: The report instance to add context to
+        model_instance: The model instance which initiated the report generation
+        request: The request object which initiated the report generation
+        context: The context dictionary to add to
+    """
+
+    # Implemlented in the specific plugin code
+    ...
+"""
+```
+
 ### Example
 
 A sample plugin which provides additional context data to the report templates can be found [in the InvenTree source code](https://github.com/inventree/InvenTree/blob/master/InvenTree/plugin/samples/integration/report_plugin_sample.py):
@@ -33,7 +55,7 @@ class SampleReportPlugin(ReportMixin, InvenTreePlugin):
         """Some custom function which is not required for the plugin to function"""
         return random.randint(0, 100)
 
-    def add_report_context(self, instance, request, context):
+    def add_report_context(self, report_instance, model_instance, request, context):
         """Add example content to the report instance"""
 
         # We can add any extra context data we want to the report
@@ -45,7 +67,7 @@ class SampleReportPlugin(ReportMixin, InvenTreePlugin):
         context['random_int'] = self.some_custom_function()
 
         # We can also add extra data to the context which is specific to the report type
-        context['is_purchase_order'] = isinstance(instance, PurchaseOrderReport)
+        context['is_purchase_order'] = isinstance(report_instance, PurchaseOrderReport)
 
         # We can also use the 'request' object to add extra context data
         context['request_method'] = request.method