auto merge of #18880 : barosl/rust/doc-fail-to-panic, r=alexcrichton

I found some occurrences of "failure" and "fails" in the documentation. I changed them to "panics" if it means a task panic. Otherwise I left it as is, or changed it to "errors" to clearly distinguish them.

Also, I made a minor fix that is breaking the layout of a module page. "Example" is shown in an irrelevant place from the following page: http://doc.rust-lang.org/std/os/index.html

src/libstd/io/mod.rs

@@ -914,9 +914,9 @@ impl<'a> Reader for &'a mut Reader+'a {
 /// Similar to `slice()` except this function only bounds the slice on the
 /// capacity of `v`, not the length.
 ///
-/// # Failure
+/// # Panics
 ///
-/// Fails when `start` or `end` point outside the capacity of `v`, or when
+/// Panics when `start` or `end` point outside the capacity of `v`, or when
 /// `start` > `end`.
 // Private function here because we aren't sure if we want to expose this as
 // API yet. If so, it should be a method on Vec.