auto merge of #17366 : ohazi/rust/master, r=steveklabnik

See: http://doc.rust-lang.org/std/from_str/trait.FromStr.html
```
let input_num = from_str::<Option<uint>>("5");
```
```
<anon>:2:21: 2:45 error: failed to find an implementation of trait std::from_str::FromStr for core::option::Option<uint>
<anon>:2     let input_num = from_str::<Option<uint>>("5");
                             ^~~~~~~~~~~~~~~~~~~~~~~~
```

Author: bors

src/doc/guide.md

@@ -2151,14 +2151,10 @@ In this case, we say `x` is a `uint` explicitly, so Rust is able to properly
 tell `random()` what to generate. In a similar fashion, both of these work:
 
 ```{rust,ignore}
-let input_num = from_str::<Option<uint>>("5");
+let input_num = from_str::<uint>("5");
 let input_num: Option<uint> = from_str("5");
 ```
 
-In this case, I happen to prefer the latter, and in the `random()` case, I prefer
-the former. I think the nested `<>`s make the first option especially ugly and
-a bit harder to read.
-
 Anyway, with us now converting our input to a number, our code looks like this:
 
 ```{rust,ignore}