kvm: x86: Amend the KVM_GET_SUPPORTED_CPUID API documentation

Document the subtle nuances that KVM_CAP_X86_DISABLE_EXITS induces in
the KVM_GET_SUPPORTED_CPUID API.

Fixes: 4d5422cea3 ("KVM: X86: Provide a capability to disable MWAIT intercepts")
Signed-off-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

Documentation/virtual/kvm/api.txt

@@ -1269,12 +1269,18 @@ struct kvm_cpuid_entry2 {
 	__u32 padding[3];
 };
 
-This ioctl returns x86 cpuid features which are supported by both the hardware
-and kvm.  Userspace can use the information returned by this ioctl to
-construct cpuid information (for KVM_SET_CPUID2) that is consistent with
-hardware, kernel, and userspace capabilities, and with user requirements (for
-example, the user may wish to constrain cpuid to emulate older hardware,
-or for feature consistency across a cluster).
+This ioctl returns x86 cpuid features which are supported by both the
+hardware and kvm in its default configuration.  Userspace can use the
+information returned by this ioctl to construct cpuid information (for
+KVM_SET_CPUID2) that is consistent with hardware, kernel, and
+userspace capabilities, and with user requirements (for example, the
+user may wish to constrain cpuid to emulate older hardware, or for
+feature consistency across a cluster).
+
+Note that certain capabilities, such as KVM_CAP_X86_DISABLE_EXITS, may
+expose cpuid features (e.g. MONITOR) which are not supported by kvm in
+its default configuration. If userspace enables such capabilities, it
+is responsible for modifying the results of this ioctl appropriately.
 
 Userspace invokes KVM_GET_SUPPORTED_CPUID by passing a kvm_cpuid2 structure
 with the 'nent' field indicating the number of entries in the variable-size