Documentation: x86: Contiguous cbm isn't all X86

Since commit 4d05bf71 ("x86/resctrl: Introduce AMD QOS feature")
resctrl has supported non-contiguous cache bit masks. The interface
for this is currently try-it-and-see.

Update the documentation to say Intel CPUs have this requirement,
instead of X86.

Cc: Babu Moger <Babu.Moger@amd.com>
Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>