Documentation/x86: Fix PR_SET_SPECULATION_CTRL error codes

Documentation/x86: Fix PR_SET_SPECULATION_CTRL error codes

If you force-disable mitigations on the kernel cmdline, for SPEC_STORE_BYPASS
this ends up with the prctl returning -ENXIO, but contrary to the current docs
for the other controls it returns -EPERM. Fix that.

Note that this return value should probably be considered a bug. But, making
the behaviour consistent with the current docs seems more likely to break
existing users than help anyone out in practice, so just "fix" it by
specifying it as correct.

Since this is getting more wordy and confusing, also be more explicit about
"control is not possible" be mentioning the boot configuration, to better
distinguish this case conceptually from the FORCE_DISABLE failure mode.

Signed-off-by: Brendan Jackman <jackmanb@google.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://patch.msgid.link/20251111-b4-prctl-docs-2-v2-1-bc9d14ec9662@google.com