doc: document time() and timeEnd() relationship

`console.time()` and `console.timeEnd()` are very closely related. It's
useful to reference them both from each other.

Previously, console.time() did not mention that it needed to be paired
with a call to console.timeEnd() to be useful, and timeEnd() also failed
to mention that console.time() needed to be called first.

References in both directions have been added.

PR-URL: https://github.com/iojs/io.js/pull/198
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>