Update docs for precache

[wibblymat]

trafficstars

The feature is now more awesome, so I should say that in the docs.

[jeffposnick]

What are you referring to, exactly? (Feel free to explain it here and then copy your explanation into the docs. :smile:)

[gauntface]

Should this be closed now?

[wibblymat]

So, this refers to the fact that the docs say precache takes an array of strings, but it actually works with promises too. Docs are still out of date.