[Bug 263958] handbook/cutting-edge: Added info for re-using /usr/obj for builds

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=263958

            Bug ID: 263958
           Summary: handbook/cutting-edge: Added info for re-using
                    /usr/obj for builds
           Product: Documentation
           Version: Latest
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: Documentation
          Assignee: doc@FreeBSD.org
          Reporter: jon@xyinn.org

Created attachment 233898
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=233898&action=edit
[patch] Added info for re-using /usr/obj for builds

Started as an observation here:
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=263880

I've added a bit more info to the "24.6.4.1. Performing a Clean Build" section
since at the moment it reads as if re-using /usr/obj is the default and you
have to explicitly clean up. On my FreeBSD 13.1-STABLE box (and also
14-CURRENT, and also stated here:
https://forums.freebsd.org/threads/make-buildworld-speed-up-possibilities.73127/#post-445913)
I've observed that make will clean up automatically before doing 'buildworld
buildkernel'. Adding the -DNO_CACHE option or specifying "WITHOUT_CLEAN=" in
/etc/src.conf does what we want (re-using /usr/obj and speeding up builds).

I've also added a warning (based on my previous experience running Gentoo and
ccache) letting the user know that re-using build objects could lead to
situations where there are bugs/instabilities/weirdness and that if in doubt,
one should clean world and compile/install fresh sources.

Since I'm not familiar with building FreeBSD pre v13, someone more experienced
than me can confirm and probably help me rewrite this part "Some versions of
the FreeBSD build system leave previously-", where "some" is what I don't know,
thus I specified 13 or later explicitly. Also if you are confident that there
wouldn't be any weirdness with re-using the /usr/obj build objects, we can drop
the warning all together.

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