Clarify that configuration_data()'s copy is immutable

docs/markdown/Configuration.md

@@ -39,6 +39,7 @@ use a single `configuration_data` object as many times as you like,
 but it becomes immutable after being passed to the `configure_file`
 function. That is, after it has been used once to generate output the
 `set` function becomes unusable and trying to call it causes an error.
+Copy of immutable `configuration_data` is still immutable.
 
 For more complex configuration file generation Meson provides a second
 form. To use it, put a line like this in your configuration file.

test cases/failing/75 configuration immutable/meson.build

@@ -0,0 +1,12 @@
+project('configuration_data is immutable')
+
+a = configuration_data()
+
+configure_file(
+    configuration : a,
+    input : 'input',
+    output : 'output',
+)
+
+still_immutable = a
+still_immutable.set('hello', 'world')