kconfig: add hints/tips/tricks to Documentation/kbuild/kconfig-language.txt

Add a section on kconfig hints: how to do <something> in Kconfig files.

Fix a few typos/spellos.

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