Documentation/arch/x86: Hide clearcpuid=

Documentation/arch/x86: Hide clearcpuid=

This option was never meant to be used in production because it solely
clears the X86_FEATURE kernel-internal representation of what CPUID bits
it has detected and doesn't do any *proper* feature disablement like
clearing CR4.CET in the user shadow stack case, for example.

So remove its documentation so that it doesn't get used in production
and people get silly ideas. It is meant strictly for debugging; and if
a chicken bit for properly disabling a feature is warranted, then that
would need proper enablement.

No functional changes.

Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Mathias Krause <minipli@grsecurity.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://patch.msgid.link/20260520202508.160112-1-bp@kernel.org