ports.7: Add the DEVELOPER make variable

make.conf(5) says make variables that are for the ports tree should
be documented in ports(7). Add this from the porters handbook.

MFC after:		3 days
Reviewed by:		alven
Event:			FreeBSD 2026 Vendor Summit
Differential Revision:	https://reviews.freebsd.org/D61272

share/man/man7/ports.7

@@ -25,7 +25,7 @@
 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 .\"
-.Dd October 7, 2026
+.Dd November 6, 2026
 .Dt PORTS 7
 .Os
 .Sh NAME
@@ -576,6 +576,9 @@ and the
 .Fx
 Porter's Handbook.
 .Bl -tag -width "WITHOUT_TESTING_PORTS"
+.It Va DEVELOPER
+If set to yes, display deprecation warnings and
+some quality checks on calling make.
 .It Va WITH_DEBUG
 .Pq Vt bool
 If set, debugging symbols are installed for ports binaries.