doc: native module reloading is not supported

Clarify in docs for require.cache that reloading native modules
isn't supported.

Related: #6160
PR-URL: https://github.com/nodejs/node/pull/6168


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: James M Snell <jasnell@gmail.com>