doc: createServer's key option can be an array

The `tls` module's `createServer` and `createSecureContext` accept
`key` option and it can be an array of keys as well. This patch
explains the format of the entries in that array.

Corresponding code:
https://github.com/nodejs/node/blob/v4.1.1/lib/_tls_common.js#L73-L90

PR-URL: https://github.com/nodejs/node/pull/3123


Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>