docs: admin-guide: update description for kernel.modprobe sysctl

When I added CONFIG_MODPROBE_PATH, I neglected to update Documentation/.
It's still true that this defaults to /sbin/modprobe, but now via a level
of indirection.  So document that the kernel might have been built with
something other than /sbin/modprobe as the initial value.

Link: https://lkml.kernel.org/r/20210420125324.1246826-1-linux@rasmusvillemoes.dk
Fixes: 17652f42 ("modules: add CONFIG_MODPROBE_PATH")
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jessica Yu <jeyu@kernel.org>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/admin-guide/sysctl/kernel.rst

@@ -483,10 +483,11 @@ modprobe
 ========
 
 The full path to the usermode helper for autoloading kernel modules,
-by default "/sbin/modprobe".  This binary is executed when the kernel
-requests a module.  For example, if userspace passes an unknown
-filesystem type to mount(), then the kernel will automatically request
-the corresponding filesystem module by executing this usermode helper.
+by default ``CONFIG_MODPROBE_PATH``, which in turn defaults to
+"/sbin/modprobe".  This binary is executed when the kernel requests a
+module.  For example, if userspace passes an unknown filesystem type
+to mount(), then the kernel will automatically request the
+corresponding filesystem module by executing this usermode helper.
 This usermode helper should insert the needed module into the kernel.
 
 This sysctl only affects module autoloading.  It has no effect on the