Initially, I wanted to add it directly to the documentation of `str. parse()' method, I finally found that it was more relevant (I hope so?) to directly document the structure in question. I've added a scenario, in which we could all get caught at least once, to make it easier to diagnose the problem when parsing integers.

The error was:

```
[00:05:25] tidy error: /checkout/src/libcore/num/mod.rs:3848: trailing whitespace
[00:05:25] tidy error: /checkout/src/libcore/num/mod.rs:3851: line longer than 100 chars
[00:05:25] tidy error: /checkout/src/libcore/num/mod.rs:3851: trailing whitespace
[00:05:26] some tidy checks failed
```
The line was truncated to 92 characters.