Specify API checks for tangle_group runs in if-else

Signed-off-by: Larsen, Steffen <[email protected]>

test_plans/non_uniform_groups.asciidoc

@@ -191,9 +191,11 @@ Check that `leader()` return type is `bool` and return value is equal to
 
 === The `tangle_group` class API
 
-The `get_tangle_group` is called with the `sub_group` of the invocation. This
-will only be called by the first `N` items of the sub-group, where `N` is
-strictly less than the result of `get_local_range()` on the sub-group.
+The `get_tangle_group` is called with the `sub_group` of the invocation. 
+Let `M` be the result of `get_local_range()` on this `sub_group` and let `N` be
+some value strictly less than `M`. `get_tangle_group` is called it two split
+control-flows in an if-else-statement, the if-branch with the first `N` items of
+the sub-group and the else branch with the rest.
 
 ==== Group traits