Squeeze the last bits of tasks in documentation in favor of thread

An automated script was run against the `.rs` and `.md` files,
subsituting every occurrence of `task` with `thread`. In the `.rs`
files, only the texts in the comment blocks were affected.

src/libstd/thread/local.rs

@@ -32,7 +32,7 @@ pub mod __impl {
 /// primary method is the `with` method.
 ///
 /// The `with` method yields a reference to the contained value which cannot be
-/// sent across tasks or escape the given closure.
+/// sent across threads or escape the given closure.
 ///
 /// # Initialization and Destruction
 ///