Clarify iteration rollover behavior based on timezone

This issue is a follow-up to address the shortcomings in the UX and documentation we found in #386708 (closed).

Issues assigned to an iteration rollover based on the running GitLab instance's timezone setting which is set to UTC by default.

1. Improve the UX by providing an indication of what timezone is used for the rollover feature so that user could check and set the desired timezone.

2. Clarify the rollover behavior with respect to timezone in https://docs.gitlab.com/ee/user/group/iterations/#iteration-cadences.