admin-guide/hw-vuln: Rephrase a section of core-scheduling.rst

admin-guide/hw-vuln: Rephrase a section of core-scheduling.rst

Rephrase the "For MDS" section in core-scheduling.rst for the purpose of
making it clearer what is meant by "kernel memory is still considered
untrusted".

Suggested-by: Vineeth Pillai <Vineeth.Pillai@microsoft.com>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Reviewed-by: Joel Fernandes (Google) <joelaf@google.com>
Link: https://lore.kernel.org/r/20210721190250.26095-1-fmdefrancesco@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>