Documentation for the ports tree includes:

ports(7)		https://man.openbsd.org/ports.7
packages(7)		https://man.openbsd.org/packages.7
mirroring-ports(7)	https://man.openbsd.org/mirroring-ports.7
library-specs(7)	https://man.openbsd.org/library-specs.7
bsd.port.mk(5)		https://man.openbsd.org/bsd.port.mk.5
bsd.port.arch.mk(5)	https://man.openbsd.org/bsd.port.arch.mk.5
port-modules(5)		https://man.openbsd.org/port-modules.5

And for bulk builds:

dpb(1)			https://man.openbsd.org/dpb.1
bulk(8)			https://man.openbsd.org/bulk.8
proot(1)		https://man.openbsd.org/proot.1

Also see the OpenBSD Porter's Handbook
https://www.openbsd.org/faq/ports/