Document how to use sudo for SU_CMD

It is rather common for the ports users to replace su(1) with sudo(8)
within the SU_CMD variable. Let's document it in the manual page (so far
it's been hidden in a comment within bsd.commands.mk).

MFC after:	2 weeks

share/man/man7/ports.7

@@ -25,7 +25,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd August 3, 2020
+.Dd November 1, 2020
 .Dt PORTS 7
 .Os
 .Sh NAME
@@ -442,7 +442,10 @@ The unprivileged user must have write access to
 and
 .Va DISTDIR .
 The default is
-.Ql /usr/bin/su root -c
+.Ql /usr/bin/su root -c .
+Many users set it to
+.Ql /usr/local/bin/sudo -E sh -c
+for convenience.
 .It Va PACKAGES
 Used only for the
 .Cm package