Create documentation for the manual sorting / re-ordering capability

doc/user/project/issue_board.md

@@ -386,6 +386,10 @@ a given board inside your GitLab instance, any time those two issues are subsequ
 loaded in any board in the same instance (could be a different project board or a different group board, for example),
 that ordering will be maintained.
 
+This ordering also affects [issue lists](issues/sorting_issue_lists.md).
+Changing the order in an issue board changes the ordering in an issue list,
+and vice versa.
+
 ### Filtering issues
 
 You should be able to use the filters on top of your Issue Board to show only

doc/user/project/issues/index.md

@@ -104,7 +104,8 @@ view, you can also make certain changes [in bulk](../bulk_editing.md) to the dis
 For more information, see the [Issue Data and Actions](issue_data_and_actions.md) page
 for a rundown of all the fields and information in an issue.
 
-For sorting by issue priority, see [Label Priority](../labels.md#label-priority).
+You can sort a list of issues several ways, including by issue creation date, milestone due date,
+etc. For more information, see the [Sorting and Ordering Issue Lists](sorting_issue_lists.md) page.
 
 ### Issue boards
 

doc/user/project/issues/sorting_issue_lists.md

+# Sorting and Ordering Issue Lists
+
+You can sort a list of issues several ways, including by issue creation date, milestone due date,
+etc. The available sorting options can change based on the context of the list.
+For sorting by issue priority, see [Label Priority](../labels.md#label-priority).
+
+In group and project issue lists, it is also possible to order issues manually,
+similar to [issue boards](../issue_board.md#issue-ordering-in-a-list).
+
+## Manual sorting
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab-ce/issues/62178) in GitLab 12.1.
+
+When you select **Manual** sorting, you can change
+the order by dragging and dropping the issues. The changed order will persist. Everyone who visits the same list will see the reordered list, with some exceptions.
+
+Each issue is assigned a relative order value, representing its relative
+order with respect to the other issues in the list. When you drag-and-drop reorder 
+an issue, its relative order value changes accordingly.
+
+In addition, any time that issue appears in a manually sorted list,
+the updated relative order value will be used for the ordering.  This means that
+if issue `A` is drag-and-drop reordered to be above issue `B` by any user in
+a given list inside your GitLab instance, any time those two issues are subsequently
+loaded in any list in the same instance (could be a different project issue list or a 
+different group issue list, for example), that ordering will be maintained.
+
+This ordering also affects [issue boards](../issue_board.md#issue-ordering-in-a-list).
+Changing the order in an issue list changes the ordering in an issue board,
+and vice versa.