Merge branch 'winh-issue-board-switcher-docs' into 'master'

Documentation for search box in issue boards switcher

See merge request gitlab-org/gitlab-ce!24246

(cherry picked from commit ba5a9033)

7872202c Document search box in issue boards switcher
623db430 Replace issue_boards_multiple.png after adding search box

doc/user/project/issue_board.md

@@ -175,6 +175,7 @@ products.
 Clicking on the current board name in the upper left corner will reveal a
 menu from where you can create another Issue Board and rename or delete the
 existing one.
+Using the search box at the top of the menu, you can filter the listed boards.
 When you're revisiting an issue board in a project or group with multiple boards, 
 GitLab will automatically load the last board you visited.