services: qemu-binfmt: 'guix-support?' defaults to #t.

* gnu/services/virtualization.scm (qemu-binfmt-service-type)[guix-support?]:
Change the default from #f to #t.
* doc/guix.texi (Transparent Emulation with QEMU): Change the default of
‘guix-support?’ from #f to #t.  Describe the implication of setting it to #f.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>

doc/guix.texi

@@ -27338,7 +27338,7 @@ This is the configuration for the @code{qemu-binfmt} service.
 The list of emulated QEMU platforms.  Each item must be a @dfn{platform
 object} as returned by @code{lookup-qemu-platforms} (see below).
 
-@item @code{guix-support?} (default: @code{#f})
+@item @code{guix-support?} (default: @code{#t})
 When it is true, QEMU and all its dependencies are added to the build
 environment of @command{guix-daemon} (@pxref{Invoking guix-daemon,
 @option{--chroot-directory} option}).  This allows the @code{binfmt_misc}
@@ -27367,6 +27367,10 @@ build}, transparently using QEMU to emulate the ARMv7 CPU.  Pretty handy
 if you'd like to test a package build for an architecture you don't have
 access to!
 
+When @code{guix-support?} is set to @code{#f}, programs for other
+architectures can still be executed transparently, but invoking commands
+like @command{guix build -s armhf-linux @dots{}} will fail.
+
 @item @code{qemu} (default: @code{qemu})
 The QEMU package to use.
 @end table

gnu/services/virtualization.scm

@@ -717,7 +717,7 @@ potential infinite waits blocking libvirt."))
   (platforms   qemu-binfmt-configuration-platforms
                (default '()))                     ;safest default
   (guix-support? qemu-binfmt-configuration-guix-support?
-                 (default #f)))
+                 (default #t)))
 
 (define (qemu-platform->binfmt qemu platform)
   "Return a gexp that evaluates to a binfmt string for PLATFORM, using the