Document htmlSafe

`htmlSafe` is useful, but undocumented.

![String docs](http://f.cl.ly/items/2p2347391v3o3U1F0F00/Screen%20Shot%202013-07-19%20at%209.50.16%20AM.png)

![Static docs](http://f.cl.ly/items/1M1H0m362J2r0t2K1z0k/Screen%20Shot%202013-07-19%20at%209.49.27%20AM.png)