doc: add missing added: tag for zlib.constants

PR-URL: #10826
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: James M Snell <[email protected]>

Author: addaleax

doc/api/zlib.md

@@ -388,6 +388,9 @@ Reset the compressor/decompressor to factory defaults. Only applicable to
 the inflate and deflate algorithms.
 
 ## zlib.constants
+<!-- YAML
+added: v7.0.0
+-->
 
 Provides an object enumerating Zlib-related constants.