handbook/cutting-edge: Add more details about MAKEOBJDIRPREFIX

Previously, the handbook suggested /tmp for MAKEOBJDIRPREFIX. This was
fine, but suggesting /tmp/obj instead is even better as build artifacts
will not mix with other files in /tmp.  Also note that MAKEOBJDIRPREFIX
must exist.

Sponsored by:	Klara, Inc.

documentation/content/en/books/handbook/cutting-edge/_index.adoc

@@ -1179,10 +1179,11 @@ For example, the following `tools/build/make.py` invocation builds the world:
 
 [source,shell]
 ....
-MAKEOBJDIRPREFIX=/tmp tools/build/make.py -j 8 TARGET=arm64 TARGET_ARCH=aarch64 buildworld
+MAKEOBJDIRPREFIX=/tmp/obj tools/build/make.py -j 8 TARGET=arm64 TARGET_ARCH=aarch64 buildworld
 ....
 
-It builds the world for target `aarch64:arm64` on 8 CPUs and uses [.filename]#/tmp# for object files.
+It builds the world for target `aarch64:arm64` on 8 CPUs and uses [.filename]#/tmp/obj# for object files.
 Note that the variables `MAKEOBJDIRPREFIX`, `TARGET`, and `TARGET_ARCH` are mandatory when building on non-FreeBSD hosts.
+Also, make sure to create the object directory pointed to by the `MAKEOBJDIRPREFIX` environment variable.
 
 Refer to man:arch[7] and man:build[7] for more details.