Document mocking issue with context-aware interfaces

Author: fbiville

MIGRATION_GUIDE.md

@@ -367,6 +367,14 @@ are all internal to the driver.
 This should have 0 impact as they were internal utilities unreachable from
 the public APIs.
 
+### Private functions in interfaces
+
+Both `neo4j.SessionWithContext` and `neo4j.ResultWithContext` define unexported functions.
+These are needed to avoid duplication between the `context.Context`-aware interface and their legacy, pre v5, variants.
+This may, however, cause issues with mocking libraries based on code generation.
+
+It is possible to mock these interfaces, as explained [here](https://groups.google.com/g/golang-nuts/c/6hpUErAfMHI).
+
 ### Behavior changes
 
 #### `ResultSummary#ContainUpdates`