Documentation: kvm: clarify KVM_SET_USER_MEMORY_REGION

The documentation does not mention how to delete a slot, add the
information.

Reported-by: Nathaniel McCallum <[e-mail]>
Signed-off-by: Paolo Bonzini <[e-mail]>

Documentation/virtual/kvm/api.txt

···
 #define KVM_MEM_LOG_DIRTY_PAGES	(1UL << 0)
 #define KVM_MEM_READONLY	(1UL << 1)
 
-This ioctl allows the user to create or modify a guest physical memory
-slot.  When changing an existing slot, it may be moved in the guest
-physical memory space, or its flags may be modified.  It may not be
-resized.  Slots may not overlap in guest physical address space.
-Bits 0-15 of "slot" specifies the slot id and this value should be
-less than the maximum number of user memory slots supported per VM.
-The maximum allowed slots can be queried using KVM_CAP_NR_MEMSLOTS,
-if this capability is supported by the architecture.
+This ioctl allows the user to create, modify or delete a guest physical
+memory slot.  Bits 0-15 of "slot" specify the slot id and this value
+should be less than the maximum number of user memory slots supported per
+VM.  The maximum allowed slots can be queried using KVM_CAP_NR_MEMSLOTS,
+if this capability is supported by the architecture.  Slots may not
+overlap in guest physical address space.
 
 If KVM_CAP_MULTI_ADDRESS_SPACE is available, bits 16-31 of "slot"
 specifies the address space which is being modified.  They must be
···
 KVM_CAP_MULTI_ADDRESS_SPACE capability.  Slots in separate address spaces
 are unrelated; the restriction on overlapping slots only applies within
 each address space.
+
+Deleting a slot is done by passing zero for memory_size.  When changing
+an existing slot, it may be moved in the guest physical memory space,
+or its flags may be modified, but it may not be resized.
 
 Memory for the region is taken starting at the address denoted by the
 field userspace_addr, which must point at user addressable memory for