doc: note that fs.futimes only works on AIX >7.1

PR-URL: https://github.com/nodejs/node/pull/13659
Fixes: https://github.com/nodejs/node/issues/12609
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com> i

doc/api/fs.md

@@ -979,6 +979,9 @@ added: v0.4.2
 Change the file timestamps of a file referenced by the supplied file
 descriptor.
 
+*Note*: This function does not work on AIX versions before 7.1, it will return
+the error `UV_ENOSYS`.
+
 ## fs.futimesSync(fd, atime, mtime)
 <!-- YAML
 added: v0.4.2