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

Documentation/kselftest.txt

 Linux Kernel Selftests
 
 The kernel contains a set of "self tests" under the tools/testing/selftests/
-directory. These are intended to be small unit tests to exercise individual
-code paths in the kernel.
+directory. These are intended to be small tests to exercise individual code
+paths in the kernel. Tests are intended to be run after building, installing
+and booting a kernel.
 
 On some systems, hot-plug tests could hang forever waiting for cpu and
 memory to be ready to be offlined. A special hot-plug target is created