doc: add documentation for fs.WriteStream.close()

PR-URL: https://github.com/nodejs/node/pull/38610
Fixes: https://github.com/nodejs/node/issues/38606


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>