[PATCH qemu] docs/interop/qcow2.txt: fix description about "zlib" clusters

[~akihirosuda]

From: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

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

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>
---
 docs/interop/qcow2.txt | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt
index f7dc304ff6..e7f036c286 100644
--- a/docs/interop/qcow2.txt
+++ b/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 ===
 
-- 
2.38.4