DOC: clarify that compression works for HTTP/2

This patch clarifies that compression also works with HTTP/2. I have
picked the wording "HTTP/1.1 or above" because it is already used
elsewhere in the documentation.

I have tested that compression indeed works in HTTP/2.

Signed-off-by: Julien Pivotto <roidelapluie@inuits.eu>
diff --git a/doc/configuration.txt b/doc/configuration.txt
index edf7c54..68df53a 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -4263,7 +4263,7 @@
   Compression is disabled when:
     * the request does not advertise a supported compression algorithm in the
       "Accept-Encoding" header
-    * the response message is not HTTP/1.1
+    * the response message is not HTTP/1.1 or above
     * HTTP status code is not one of 200, 201, 202, or 203
     * response contain neither a "Content-Length" header nor a
       "Transfer-Encoding" whose last value is "chunked"