devguide: complement info on branches to work on

With the change from master to main, ensure that we still list examples
of stable branches, as the text also refers to those.

doc/userguide/devguide/codebase/contributing/contribution-process.rst

@@ -153,9 +153,10 @@ What branch to work on
 
 There are 2 or 3 active branches:
 
+    * main-x.x.x (e.g. main-7.0.x)
     * main
 
-The former is the stable branch. The latter the development branch.
+The former is a stable branch. The latter is the development branch.
 
 The stable branch should only be worked on for important bug fixes. Those are
 mainly expected from more experienced contributors.