R1:8ffa6044626d

Diviner: mention how to flag Arcanist and Phorge as "safe" git repos

Summary:
This tip is useful if you are affected by the following error in git or similar:

fatal: detected dubious ownership in repository at '/var/www/phorge'

Just follow the new tip from Diviner to fix at system level.

Ref T15282

Test Plan:
- see if you like the documentation tip
- try to execute the command, and verify that your computer does not explode
- eventually visit the /config/ page with and without that change (with…