doc: dev-tools: kselftest.rst: update to include make O=dir support

Update to include details on make O=dir support and other changes improve
test results output.

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>
[jc: Tweaked RST formatting slightly ]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>