docs: Rephrase the LVM limitation

Make sure the user understands that this limitation has to do with the
input media internals and not with the place where the input media
itself is stored.

docs/usage.rst

@@ -314,8 +314,8 @@ them or perform system preparation operations.
 Logical Volumes
 ---------------
 
-The program cannot work on LVM partitions [#f1]_. The input media may only
-contain primary or logical partitions.
+The program cannot work on input media that contain LVM partitions inside
+[#f1]_. The input media may only contain primary or logical partitions.
 
 Para-virtualized drivers
 ------------------------