You don’t see lanx, because they are not present on TOS 3. In TOS 3, you have CPU with it’s interfaces and then you configure switch via switchconfig while looking at which ports are connected where and manually passing it to ports connected to CPU. In TOS 5, you don’t configure switch explicitely. Kernel does that for you. You will have lanx interfaces representing physical ports on the box and you can setup vlans and bridges as normal with those interfaces. And kernel will figure out switch and CPU ports configuration for you. It is no longer needed to specify it by hand and you get interface for every port directly making it much easier to manage.
Docs are usually updated to be up to date with the latest version. There is currently TOS 5 based on OpenWrt 19.07, but the latest OpenWrt docs might already be for recently released OpenWrt 20.02 - TOS 6 that will take us some time to prepare. But mostly I found that where options were version dependent, it was explicitelly mentioned.