build.7: Document pkgbase PKG_FORMAT variable

MFC after: 3 days
Reported by: emaste
Reviewed by: emaste, mhorne
Approved by: mhorne (mentor)
Differential Revision: https://reviews.freebsd.org/D50318

(cherry picked from commit dd3d1a7694583362f5915ff5f1d9cdee64e2fe3e)