Documentation: kunit: Update kunit_tool page

The kunit_tool documentation page was pretty minimal, and a bit
outdated. Update it and flesh it out a bit.

In particular,
- Mention that .kunitconfig is now in the build directory
- Describe the use of --kunitconfig to specify a different config
  framgent
- Mention the split functionality (i.e., commands other than 'run')
- Describe --raw_output and kunit.py parse
- Mention the globbing support
- Provide a quick overview of other options, including --build_dir and
  --alltests

Note that this does overlap a little with the new running_tips page. I
don't think it's a problem having both: this page is supposed to be a
bit more of a reference, rather than a list of useful tips, so the fact
that they both describe the same features isn't a problem.

Signed-off-by: David Gow <davidgow@google.com>
Reviewed-by: Daniel Latypov <dlatypov@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>