kselftest: kselftest documentation improvement

Suggested patch solves two issues:

1) Currently documentation is unclear whether `make kselftest` should
be run before or after kernel was installed and booted. `make help`
gives a clear answer on that: "kselftest - Build and run kernel selftest
(run as root). Build, install, and boot kernel before running kselftest
on it."

2) Documentation states that `make kselftest` executes "unit" tests.
Technically it's not a _unit_ test if it requires to install an
application first. It's either integration or system test. To not to
confuse a user I suggest not to use a word "unit".

Signed-off-by: Aleksander Alekseev <afiskon@devzen.ru>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>