doc: add docs for Zlib#close()

PR-URL: #16592
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

doc/api/zlib.md

@@ -363,6 +363,13 @@ added: v0.5.8
 Not exported by the `zlib` module. It is documented here because it is the base
 class of the compressor/decompressor classes.
 
+### zlib.close([callback])
+<!-- YAML
+added: v0.9.4
+-->
+
+Close the underlying handle.
+
 ### zlib.flush([kind], callback)
 <!-- YAML
 added: v0.5.8