build: note that MAKEOBJDIRPREFIX must be absolute

PR:		282711
Reviewed by:	imp
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D47538

share/man/man7/build.7

@@ -25,7 +25,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd June 4, 2024
+.Dd November 13, 2024
 .Dt BUILD 7
 .Os
 .Sh NAME
@@ -512,6 +512,8 @@ and not via
 or
 .Pa /etc/src.conf
 or the command line.
+.Va MAKEOBJDIRPREFIX
+must specify an absolute path.
 .It Va WITHOUT_WERROR
 If defined, compiler warnings will not cause the build to halt,
 even if the makefile says otherwise.