UPDATING: clarify ino64 upgrade instructions even further

- mention COMPAT_FREEBSD11 earlier so that the steps are in chronological
  order
- suggest removing /usr/obj before build to ensure there are no stale
  objects

Reviewed by:	allanjude, kib
Sponsored by:	The FreeBSD Foundation

UPDATING

@@ -53,12 +53,19 @@ NOTE TO PEOPLE WHO THINK THAT FreeBSD 12.x IS SLOW:
 
 20170523:
 	The "ino64" 64-bit inode project has been committed, which extends
-	a number of types to 64 bits.  In order to upgrade, carefully follow
-	the full procedure documented below under the heading "To rebuild
-	everything and install it on the current system."  Specifically, a
-	reboot is required after installing the new kernel before installing
-	world.  In addition, if using a custom kernel configuration ensure
-	that the COMPAT_FREEBSD11 option is included.
+	a number of types to 64 bits.  Upgrading in place requires care and
+	adherence to the documented upgrade procedure.
+
+	If using a custom kernel configuration ensure that the
+	COMPAT_FREEBSD11 option is included (as during the upgrade the
+	system will be running the ino64 kernel with the existing world).
+
+	For the safest in-place upgrade begin by removing previous build
+	artifacts via "rm -rf /usr/obj/*".   Then, carefully follow the
+	full procedure documented below under the heading "To rebuild
+	everything and install it on the current system."  Specifically,
+	a reboot is required after installing the new kernel before
+	installing world.  
 
 20170424:
 	The NATM framework including the en(4), fatm(4), hatm(4), and