x86/pmu: Refine info to clarify the current support

Existing unit tests do not cover AMD pmu, nor Intel pmu that is not
architecture (on some obsolete cpu's). AMD's PMU support will be
coming in subsequent commits.

Signed-off-by: Like Xu <likexu@tencent.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20221102225110.3023543-11-seanjc@google.com