COMPAT: Require a problem description in issues

Currently some issues are just code examples without a description
**why** a change should be done. This leads to problems when
(years later) the current behaviour is questioned and no one can
remember why it was changed.

closes #14032

.github/ISSUE_TEMPLATE.md

@@ -1,9 +1,12 @@
-#### A small, complete example of the issue
+#### Code Sample, a copy-pastable example if possible
 
 ```python
 # Your code here
 
 ```
+#### Problem description
+
+[this should explain **why** the current behaviour is a problem and why the expected output is a better solution.]
 
 #### Expected Output