Additional information for time settings

Problem to solve

gitlab-ce!25381 introduces time related settings, it might be useful to explain how these settings are applied throughout the application.

Proposal

• Add some help text to /help/user/profile/preferences.md for the changes introduced in gitlab-ce!25381 to explain how Time format and Time display will affect the application.
• Add some help text to /help/user/profile/index.md to explain how time zones are applied in the application.