Add in some <hr>s for emphasis

It's not really clear here, since the example is rendered, where it
starts and ends. So let's use <hr>s to split it up.

Author: steveklabnik

src/doc/trpl/documentation.md

@@ -279,6 +279,8 @@ println!("{}", x + y);
 
 Here's an explanation, rendered:
 
+-------------------------------------------------------------------------------
+
 First, we set `x` to five:
 
 ```rust
@@ -303,8 +305,12 @@ Finally, we print the sum of `x` and `y`:
 println!("{}", x + y);
 ```
 
+-------------------------------------------------------------------------------
+
 Here's the same explanation, in raw text:
 
+-------------------------------------------------------------------------------
+
 > First, we set `x` to five:
 >
 > ```text
@@ -329,6 +335,8 @@ Here's the same explanation, in raw text:
 > println!("{}", x + y);
 > ```
 
+-------------------------------------------------------------------------------
+
 By repeating all parts of the example, you can ensure that your example still
 compiles, while only showing the parts that are relevant to that part of your
 explanation.