Documentation: x86: Remove cdpl2 unspported statement and fix capitalisation

"L2 cache does not support code and data prioritization". This isn't
true, elsewhere the document says it can be enabled with the cdpl2
mount option.

While we're here, these sample strings have lower-case code/data,
which isn't how the kernel exports them.

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