Add a DOCDISTFILES variable to point to a directory containing the
distfiles to build the docproj port during release. Prior to this, the
distfiles were copied froom ${DISTFILES}. However, if like me, you needed
to store the distfiles in some directory other than /usr/ports/distfiles,
for example /usr/docdistfiles, then when you overrode ${DISTFILES} it broke
the release build when it tried to build docproj port in the chroot'd
release area. To preserve backwards compatibility, DOCDISTFILES defaults
to the value of DISTFILES.

Ok'd by: jkh