kconfig: Add static text for search information in help menu

Add few static text to explain how one can bring up the search dialog
box by pressing the forward slash key anywhere on this interface.

Signed-off-by: Bhaskar Chowdhury <unixbhaskar@gmail.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>