Documentation : Adjust caution admonition to be less scary

On https://docs.gitlab.com/ee/administration/geo/replication/#overview, the following caution admonition exists:

Geo_Replication__GitLab_2019-01-03_11-53-37

As discussed https://gitlab.slack.com/archives/C32LCGC1H/p1546476844024800, we believe we can now reduce this caution to a notice and use less discouraging wording 😄

Edited by Rachel Nienaber