website: update to be a little more clear [GH-933]

website/source/docs/builders/qemu.html.markdown

@@ -108,7 +108,7 @@ Optional:
   the default is 10 seconds.
 
 * `disk_size` (int) - The size, in megabytes, of the hard disk to create
-  for the VM. By default, this is 40000 (40 GB).
+  for the VM. By default, this is 40000 (about 40 GB).
 
 * `disk_interface` (string) - The interface to use for the disk. Allowed
   values include any of "ide," "scsi" or "virtio." Note also that any boot

website/source/docs/builders/virtualbox-iso.html.markdown

@@ -83,7 +83,7 @@ Optional:
   the default is 10 seconds.
 
 * `disk_size` (int) - The size, in megabytes, of the hard disk to create
-  for the VM. By default, this is 40000 (40 GB).
+  for the VM. By default, this is 40000 (about 40 GB).
 
 * `floppy_files` (array of strings) - A list of files to put onto a floppy
   disk that is attached when the VM is booted for the first time. This is

website/source/docs/builders/vmware-iso.html.markdown

@@ -86,7 +86,7 @@ Optional:
 * `disk_size` (int) - The size of the hard disk for the VM in megabytes.
   The builder uses expandable, not fixed-size virtual hard disks, so the
   actual file representing the disk will not use the full size unless it is full.
-  By default this is set to 40,000 (40 GB).
+  By default this is set to 40,000 (about 40 GB).
 
 * `disk_type_id` (string) - The type of VMware virtual disk to create.
   The default is "1", which corresponds to a growable virtual disk split in