copyedits: patterns

This also puts slice patterns in nightly docs, where they belong.

src/doc/trpl/slice-patterns.md

@@ -0,0 +1,18 @@
+% Slice patterns
+
+If you want to match against a slice or array, you can use `&` with the
+`slice_patterns` feature:
+
+```rust
+#![feature(slice_patterns)]
+
+fn main() {
+    let v = vec!["match_this", "1"];
+
+    match &v[..] {
+        ["match_this", second] => println!("The second element is {}", second),
+        _ => {},
+    }
+}
+```
+