kselftest: Move the docs to the Documentation dir

Also, adjust the formatting a bit, and expand the section about using
TARGETS= on the make command line.

Signed-off-by: Tim Bird <tim.bird@sonymobile.com>
Acked-by: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>