[trivial] Placement of WIKI ddoc macro in std.conv was corrupting docs

Not sure of the root cause of this but moving it fixes this.  It was in a weird place in the source file before, does anyone know if there was a reason for that?

Here's what the "corruption" looks like:
![image](https://f.cloud.github.com/assets/74619/503371/051c3d90-bcca-11e2-9ba8-2bd6bf3a89fd.png)