docs: Make the recommendation for the backing file name position a requirement

The current version of the qcow2 specification recommends to save the backing
file name in the end of the first cluster. It follows that the backing file
name can be saved somewhere in the image, but the first cluster, which
contradicts the current QEMU implementation.

The patch makes the backing file name required to be placed after the header
extensions in the first image cluster.
Signed-off-by: NMaria Kustova <maria.k@catit.be>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

docs/specs/qcow2.txt

@@ -135,12 +135,12 @@ be stored. Each extension has a structure like the following:
 Unless stated otherwise, each header extension type shall appear at most once
 in the same image.
 
-The remaining space between the end of the header extension area and the end of
-the first cluster can be used for the backing file name. It is not allowed to
-store other data here, so that an implementation can safely modify the header
-and add extensions without harming data of compatible features that it
-doesn't support. Compatible features that need space for additional data can
-use a header extension.
+If the image has a backing file then the backing file name should be stored in
+the remaining space between the end of the header extension area and the end of
+the first cluster. It is not allowed to store other data here, so that an
+implementation can safely modify the header and add extensions without harming
+data of compatible features that it doesn't support. Compatible features that
+need space for additional data can use a header extension.
 
 
 == Feature name table ==