Update Holiday policy clarifying the language

Why is this change being made?

The instructions directing team members to change the drop-down option to "Official holidays" via the timeanddate.com site, provided most holidays a team member is eligible for but in some countries, it did not include all of the public holidays observed. I've updated the directions noting that this is for general guidance, and if public schools or government offices in their area are closed for a particular national or regional holiday, it’s recognized for GitLab team members in that location to take that time off as a public holiday as well. I've also added the leaves@domain as the POC if team members have questions.

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