Improve specs and docs

app/services/issues/update_service.rb

@@ -90,7 +90,7 @@ module Issues
 
       issue =
         if board_group_id
-          IssuesFinder.new(current_user, group_id: board_group_id).find_by(id: id)
+          IssuesFinder.new(current_user, group_id: board_group_id, include_subgroups: true).find_by(id: id)
         else
           project.issues.find(id)
         end

doc/user/project/issue_board.md

@@ -334,8 +334,7 @@ Issue Board, that is create/delete lists and drag issues around.
 >Introduced in GitLab 10.6
 
 Group issue board is analogous to project-level issue board and it is accessible at the group
-navigation level. A group-level issue board allows you to view all issues from all projects in that group
-(currently, it does not see issues from projects in subgroups). Similarly, you can only filter by group labels for these
+navigation level. A group-level issue board allows you to view all issues from all projects in that group or descendant subgroups. Similarly, you can only filter by group labels for these
 boards. When updating milestones and labels for an issue through the sidebar update mechanism, again only
 group-level objects are available.
 

spec/services/issues/update_service_spec.rb

@@ -97,11 +97,13 @@ describe Issues::UpdateService, :mailer do
         expect(issue.relative_position).to be_between(issue1.relative_position, issue2.relative_position)
       end
 
-      context 'when moving issue between issues from different projects' do
+      context 'when moving issue between issues from different projects', :nested_groups do
         let(:group) { create(:group) }
+        let(:subgroup) { create(:group, parent: group) }
+
         let(:project_1) { create(:project, namespace: group) }
         let(:project_2) { create(:project, namespace: group) }
-        let(:project_3) { create(:project, namespace: group) }
+        let(:project_3) { create(:project, namespace: subgroup) }
 
         let(:issue_1) { create(:issue, project: project_1) }
         let(:issue_2) { create(:issue, project: project_2) }