The configuration section of Arcanist's user guide explains that in order to set configuration at the user level (rather than for the local directory), arc set-config --global has to be used. However, the lastest HEAD of arcanist (rARC57f6fb59d73994d90cd94143787424ce0fdbf73b) reports that it will write user configuration by default, and --local has to be given to write for the local folder.
set-config [options] -- name value Supports: cli Sets an arc configuration option. Options are either user (apply to all arc commands you invoke from the current user) or local (apply only to the current working copy). By default, user configuration is written. Use --local to write local configuration. User values are written to '~/.arcrc' on Linux and Mac OS X, and an undisclosed location on Windows. Local values are written to an arc directory under either .git, .hg, or .svn as appropriate. --local Set a local config value instead of a user one.
BTW, how does one create a patch for the documentation?