common: Add documentation to the view KernelMemoryUsage

common: Add documentation to the view KernelMemoryUsage

I add the description and the functionalities of the new
kernel Memory Usage view.

Change-Id: I457372fd1d99c262ca35d768f83e2d53b130c4cb
Signed-off-by: Julien Daoust <julien_daoust@hotmail.com>
Reviewed-on: https://git.eclipse.org/r/69335
Reviewed-by: Genevieve Bastien <gbastien+lttng@versatic.net>
Tested-by: Genevieve Bastien <gbastien+lttng@versatic.net>
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-by: Hudson CI