KVM: fix documentation for KVM_CREATE_GUEST_MEMFD

The KVM_CREATE_GUEST_MEMFD ioctl returns a file descriptor, and is
documented as such in the description. However, the "Returns" field
in the documentation states that the ioctl returns 0 on success.
Update this to match the description.

Signed-off-by: Carlos López <clopez@suse.de>
Fixes: a7800aa ("KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for guest-specific backing memory")
Link: https://lore.kernel.org/r/20240424103317.28522-1-clopez@suse.de
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: 00xc

Documentation/virt/kvm/api.rst

@@ -6316,7 +6316,7 @@ The "flags" field is reserved for future extensions and must be '0'.
 :Architectures: none
 :Type: vm ioctl
 :Parameters: struct kvm_create_guest_memfd(in)
-:Returns: 0 on success, <0 on error
+:Returns: A file descriptor on success, <0 on error
 
 KVM_CREATE_GUEST_MEMFD creates an anonymous file and returns a file descriptor
 that refers to it.  guest_memfd files are roughly analogous to files created