build.7: Explain specifying multiple KERNCONFs

MFC after:		3 days
Reviewed by:		ivy, mhorne
Differential Revision:	https://reviews.freebsd.org/D56281

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 April 13, 2026
+.Dd April 22, 2026
 .Dt BUILD 7
 .Os
 .Sh NAME
@@ -578,11 +578,20 @@ name ${MACHINE_ARCH}-gcc<major version>.
 .It Va INSTKERNNAME
 If set, specify an alternative name to build and install for the various
 kernel make targets.
+Defaults to
+.Dq Li kernel .
 .It Va KERNCONF
-Overrides which kernel to build and install for the various kernel
-make targets.
-It defaults to
-.Cm GENERIC .
+Specify one or more space-separated kernels to build and install for the
+various kernel make targets.
+If multiple kernels are specified, the first listed kernel installs to
+.Pa /boot/${INSTKERNNAME} ,
+and subsequent kernels install to
+.Pa /boot/${INSTKERNNAME}.NAME .
+.Pp
+If unset, it defaults to GENERIC,
+except on POWER architectures,
+where it defaults to GENERIC64 for powerpc64,
+and GENERIC64LE for powerpc64le.
 .It Va KERNBUILDDIR
 Overrides the default directory to get all the opt_*.h files for
 building a kernel module.