Improve burndown documentation

The burndown chart is current stateless, issues created before milestone start date are considered
as assigned to the milestone on start_date. Issues closed before milestone start date are not accounted as closed in the burndown. For these reasons sometimes when you change milestone start date the open issues on each day of the burndown may go up or down.

Check https://gitlab.com/gitlab-org/gitlab-ee/issues/8538#note_119535034

Currently we have plans to update burndown to cover more edge cases, https://gitlab.com/gitlab-org/gitlab-ee/issues/6903, until there we should update our docs to let users now about these conditions.