doc: kvm: Fix return description of KVM_SET_MSRS

Userspace can use ioctl KVM_SET_MSRS to update a set of MSRs of guest.
This ioctl set specified MSRs one by one. If it fails to set an MSR,
e.g., due to setting reserved bits, the MSR is not supported/emulated by
KVM, etc..., it stops processing the MSR list and returns the number of
MSRs have been set successfully.
Signed-off-by: NXiaoyao Li <xiaoyao.li@intel.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Documentation/virt/kvm/api.txt

@@ -586,7 +586,7 @@ Capability: basic
 Architectures: x86
 Type: vcpu ioctl
 Parameters: struct kvm_msrs (in)
-Returns: 0 on success, -1 on error
+Returns: number of msrs successfully set (see below), -1 on error
 
 Writes model-specific registers to the vcpu.  See KVM_GET_MSRS for the
 data structures.
@@ -595,6 +595,11 @@ Application code should set the 'nmsrs' member (which indicates the
 size of the entries array), and the 'index' and 'data' members of each
 array entry.
 
+It tries to set the MSRs in array entries[] one by one. If setting an MSR
+fails, e.g., due to setting reserved bits, the MSR isn't supported/emulated
+by KVM, etc..., it stops processing the MSR list and returns the number of
+MSRs that have been set successfully.
+
 
 4.20 KVM_SET_CPUID