Require all list elements to have the same type

Author: mgorny

cep-0014.md

@@ -69,6 +69,7 @@ To benefit from autocompletion, and other LSP features in editors, we can add a
 
 The context section is a dictionary with key-value pairs that can be used for string interpolation.
 The right-hand side of the key-value pair is either a scalar (bool, number or string), or a list of scalars.
+If it is a list, all elements must have exactly the same type.
 The resulting Jinja variable has the same type as the value.
 
 The variables can reference other variables from the context section.