Reorder description for snippets in rustdoc documentation

The example code snippets for the `no_run` and `compile_fail` attributes
in the rustdoc documentation were followed by the description for the
wrong attribute.  This patch reorders the descriptions to match the code
snippets.

src/doc/rustdoc/src/documentation-tests.md

@@ -268,10 +268,10 @@ not actually pass as a test.
 # fn foo() {}
 ```
 
-`compile_fail` tells `rustdoc` that the compilation should fail. If it
-compiles, then the test will fail. However please note that code failing
-with the current Rust release may work in a future release, as new features
-are added.
+The `no_run` attribute will compile your code, but not run it. This is
+important for examples such as "Here's how to retrieve a web page,"
+which you would want to ensure compiles, but might be run in a test
+environment that has no network access.
 
 ```text
 /// ```compile_fail
@@ -280,7 +280,7 @@ are added.
 /// ```
 ```
 
-The `no_run` attribute will compile your code, but not run it. This is
-important for examples such as "Here's how to retrieve a web page,"
-which you would want to ensure compiles, but might be run in a test
-environment that has no network access.
+`compile_fail` tells `rustdoc` that the compilation should fail. If it
+compiles, then the test will fail. However please note that code failing
+with the current Rust release may work in a future release, as new features
+are added.