docs/interop/qcow2.txt: fix description about "zlib" clusters

"zlib" clusters are actually raw deflate (RFC1951) clusters without
zlib headers.

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>
Message-Id: <168424874322.11954.1340942046351859521-0@git.sr.ht>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

docs/interop/qcow2.txt

@@ -214,14 +214,18 @@ version 2.
                     type.
 
                     If the incompatible bit "Compression type" is set: the field
-                    must be present and non-zero (which means non-zlib
+                    must be present and non-zero (which means non-deflate
                     compression type). Otherwise, this field must not be present
-                    or must be zero (which means zlib).
+                    or must be zero (which means deflate).
 
                     Available compression type values:
-                        0: zlib <https://www.zlib.net/>
+                        0: deflate <https://www.ietf.org/rfc/rfc1951.txt>
                         1: zstd <http://github.com/facebook/zstd>
 
+                    The deflate compression type is called "zlib"
+                    <https://www.zlib.net/> in QEMU. However, clusters with the
+                    deflate compression type do not have zlib headers.
+
 
 === Header padding ===