CNV 11325 Adding info about preallocation disk parameter

CNV 11325 Changes based on code review

CNV 11325 Changes based on peer review

Author: Matthew Garrell

modules/virt-add-disk-to-vm.adoc

@@ -38,6 +38,8 @@ Use this procedure to add a virtual disk to a {object}.
 
 . In the *Add Disk* window, specify the *Source*, *Name*, *Size*, *Type*, *Interface*, and *Storage Class*.
 
+.. Advanced: You can enable preallocation if you use a blank disk source and require maximum write performance when creating data volumes. To do so, select the *Enable preallocation* checkbox.
+
 .. Optional: In the *Advanced* list, specify the *Volume Mode* and *Access Mode* for the virtual disk. If you do not specify these parameters, the system uses the default values from the `kubevirt-storage-class-defaults` config map.
 
 . Click *Add*.