docgen/budget: add comment for each budget line (resource and charge)

The comments are cocatenated by lines of each type, and separated by a
`|`

CHANGELOG.md

@@ -13,6 +13,7 @@ and this project adheres to
 <!-- write down unreleased development here -->
 * [docgen] add more persons choices in docgen for course: amongst requestor (if person), resources of course (if person), and PersonResource (if person);
 * [docgen] add a new context with a list of activities in course
+* [docgen] add a comment in budget lines
 
 
 ## Test releases