doc: clarify fs.mkdtemp prefix argument

Per: https://github.com/nodejs/node/issues/6142

Clarify the prefix argument.

Fixes: https://github.com/nodejs/node/issues/6142
PR-URL: https://github.com/nodejs/node/pull/6800


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Roman Klauke <romaaan.git@gmail.com>