I’m trying to edit the default symbol fields, which can apparently be found under menu Preferences > General Options. But I don’t have that. See this imgur album, which has a screenshot taken from the bundled manual compared with what I actually see.
macOS 10.13.6, KiCad 5.0.0-3-g5ebb6b6
Found it. On macOS, the options are in the expected KiCad > Preferences menu item, similar to most other Mac applications.