system: Change default kernel-arguments to '("quiet").

This improves the readability of the boot process, especially when using full
disk encryption where you're required to enter the passphrase at a particular
point.

* gnu/system.scm (<operating-system>)[kernel-arguments]: Change the default to
'("quiet").
* doc/guix.texi (operating-system Reference): Document this change.

doc/guix.texi

@@ -10461,7 +10461,7 @@ The package object of the operating system kernel to use@footnote{Currently
 only the Linux-libre kernel is supported.  In the future, it will be
 possible to use the GNU@tie{}Hurd.}.
 
-@item @code{kernel-arguments} (default: @code{'()})
+@item @code{kernel-arguments} (default: @code{'("quiet")})
 List of strings or gexps representing additional arguments to pass on
 the command-line of the kernel---e.g., @code{("console=ttyS0")}.
 

gnu/system.scm

@@ -160,7 +160,7 @@
   (kernel operating-system-kernel                 ; package
           (default linux-libre))
   (kernel-arguments operating-system-user-kernel-arguments
-                    (default '()))                ; list of gexps/strings
+                    (default '("quiet")))         ; list of gexps/strings
   (bootloader operating-system-bootloader)        ; <bootloader-configuration>
   (label operating-system-label                   ; string
          (thunked)