kconfig: Remove silentoldconfig from help and docs; fix kconfig/conf's help

As explained by Michal Marek at https://lkml.org/lkml/2011/8/31/189
silentoldconfig has become a misnomer. It has become an internal interface
so remove it from "make help" and Documentation/ to stop confusing people
using it as seen for instance at
https://chromium-review.googlesource.com/835632 Don't remove it from
kconfig/Makefile yet not to break any (other) tool using it.

On the other hand, correct and expand its description in the help of
the (internal) scripts/kconfig/conf.c

Signed-off-by: Marc Herbert <marc.herbert@intel.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>