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

Fixes: #12609

Author: gibfahn

doc/api/fs.md

@@ -1225,6 +1225,9 @@ changes:
 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)
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