kconfig: update kconfig-language text

Add kconfig-language docs for mainmenu, def_bool, and def_tristate.
Remove "requires" as a synonym of "depends on" since it was removed
from  the parser in commit 247537b9

.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>