docs: admin-guide: document the kernel.modprobe sysctl

docs: admin-guide: document the kernel.modprobe sysctl

Document the kernel.modprobe sysctl in the same place that all the other
kernel.* sysctls are documented.  Make sure to mention how to use this
sysctl to completely disable module autoloading, and how this sysctl
relates to CONFIG_STATIC_USERMODEHELPER.

[[email protected]: v5]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Eric Biggers <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Cc: Alexei Starovoitov <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Jeff Vander Stoep <[email protected]>
Cc: Jessica Yu <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Luis Chamberlain <[email protected]>
Cc: NeilBrown <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>