doc: add docs for Zlib#close()

PR-URL: https://github.com/nodejs/node/pull/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

@@ -406,6 +406,13 @@ The `zlib.bytesRead` property specifies the number of bytes read by the engine
 before the bytes are processed (compressed or decompressed, as appropriate for
 the derived class).
 
+### zlib.close([callback])
+<!-- YAML
+added: v0.9.4
+-->
+
+Close the underlying handle.
+
 ### zlib.flush([kind], callback)
 <!-- YAML
 added: v0.5.8