Reading the arcanist documentation at
https://secure.phabricator.com/book/phabricator/article/arcanist/
First thing I need is to install, so I follow the QuickStart link at the top.
https://secure.phabricator.com/diviner/find/?name=Arcanist_Quick_Start&type=article&jump=1
But this document says that detailed installation instructions are back at the previous document. It offers a link to platform specific instructions, though, so I click through to the MacOS link
https://secure.phabricator.com/diviner/find/?name=Arcanist_User_Guide%3A_Mac_OS_X&type=article&jump=1
This again refers the user back to the original page for the actual arcanist installation instructions.
After a while clicking around in circular links, and because I'm ex-FB, I stopped to look, and the installation instructions are on the 'Quick Start Guide' page below the links to platform specific instructions.
Highly confusing.