kselftest.rst: do some adjustments after ReST conversion

Do some minor adjustments after ReST conversion:

- On most documents, we use prepend a "$ " before
  command line arguments;

- Prefer to use :: on the preceding line;

- Split a multi-paragraph description as such.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Acked-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>