Remove useless doc comment for slice
r? @steveklabnik
This doc comment isn't useful so I removed it. It also "fixes" the weird rustdoc output.
Before:
![screenshot from 2016-07-08 02-04-59](https://cloud.githubusercontent.com/assets/
3050060/
16688164/
fbb321d6-451c-11e6-8d98-
53346bc6eb1e.png)
After:
![screenshot from 2016-07-08 02-05-31](https://cloud.githubusercontent.com/assets/
3050060/
16688166/
00b174e4-451d-11e6-83dd-
93ab2509f647.png)