Although there are two documentation options (DOCS and DOXYGEN), neither of them installs the full documentation. This means that menu options for a manual do not work in kicad, eeschema and pcbnew. These documents come in a separate distfile, kicad-doc-x.y.z.tar.gz. Given the size of the distribution, perhaps this could be a separate port?
A commit references this bug: Author: cmt Date: Fri Jun 21 13:52:02 UTC 2019 New revision: 504800 URL: https://svnweb.freebsd.org/changeset/ports/504800 Log: cad/kicad-doc: online help files and documentation for cad/kicad PR: 238678 Reported by: bob at eager dot cx Changes: head/cad/Makefile head/cad/kicad-doc/ head/cad/kicad-doc/Makefile head/cad/kicad-doc/distinfo head/cad/kicad-doc/pkg-descr head/cad/kicad-doc/pkg-plist
You're right (that demonstrates that you're the first one to try to use the online help). Here it is, cad/kicad-doc at ports r504800 I can't get chinese and polish language files to build, so I just hope we can do without them for the time being.