doc: kselftest: Fix KBUILD_OUTPUT usage instructions

Fix KBUILD_OUTPUT usage instructions. The current documentation is
incorrect. Update and fix outdated information about summary option.
Add a reference to kselftest wiki for additional information on the
framework and tips on writing new tests.

Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>