Documentation: KUnit: Rewrite getting started

Clarify the purpose of kunit_tool and fixed consistency issues
Signed-off-by: Harinder Singh <sharinder@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Link: https://lore.kernel.org/r/20211217044911.798817-3-sharinder@google.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>