clarify the xml example for NVDIMM more clealy

The NVDIMM backend file can be a normal file or a real device file,
Current xml example and explainations may mislead users. So add more
info about the NVDIMM related elements and update the xml examples.
Signed-off-by: NLuyao Zhong <luyao.zhong@intel.com>
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>

docs/formatdomain.html.in

@@ -8549,7 +8549,6 @@ qemu-kvm -net nic,model=? /dev/null
   <memory model='nvdimm'>
     <source>
       <path>/tmp/nvdimm</path>
-      <alignsize unit='KiB'>2048</alignsize>
     </source>
     <target>
       <size unit='KiB'>524288</size>
@@ -8560,9 +8559,10 @@ qemu-kvm -net nic,model=? /dev/null
       <readonly/>
     </target>
   </memory>
-  <memory model='nvdimm'>
+  <memory model='nvdimm' access='shared'>
     <source>
       <path>/dev/dax0.0</path>
+      <alignsize unit='KiB'>2048</alignsize>
       <pmem/>
     </source>
     <target>
@@ -8596,6 +8596,8 @@ qemu-kvm -net nic,model=? /dev/null
           module basis. Values are the same as
           <a href="#elementsMemoryBacking">Memory Backing</a>:
           <code>shared</code> and <code>private</code>.
+          For <code>nvdimm</code> model, if using real NVDIMM DAX device as
+          backend, <code>shared</code> is required.
         </p>
       </dd>
 
@@ -8659,7 +8661,8 @@ qemu-kvm -net nic,model=? /dev/null
               alignment used to mmap the address range for the backend
               <code>path</code>. If not supplied the host page size is used.
               For example, to mmap a real NVDIMM device a 2M-aligned page may
-              be required.
+              be required, and host page size is 4KB, then we need to set this
+              element to 2MB.
               <span class="since">Since 5.0.0</span>
             </p>
           </dd>