Subversion is not installed by textproc/docproj by default.
Direct reader to install the docproj package as well as the subversion package.
From the BSDCam 2017 Doc hackday.

Submitted by: Deb Goodkin <deb AT freebsdfoundation DOT org>
Approved by: bcr (mentor)
Differential Revision: https://reviews.freebsd.org/D11807