Documentation: KUnit: Reword start guide for selecting tests

Reword "Creating a ``.kunitconfig``" into "Selecting which tests to run"
covering the current alternatives for editing configs and glob-filtering

Signed-off-by: Tales Aparecida <tales.aparecida@gmail.com>
Reviewed-by: Maíra Canal <mairacanal@riseup.net>
Reviewed-by: Sadiya Kazi <Sadiyakazi@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>