Documentation: kunit: Add clang UML coverage example

LLVM-based toolchain is using a different set of tools for coverage.
Add an example that produces output in lcov format.

Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Reviewed-by: David Gow <davidgow@google.com>
[rw: Added spelling fixes from David Gow]
Signed-off-by: Richard Weinberger <richard@nod.at>