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/doc/complement-design-faq.md

@@ -96,7 +96,7 @@ code should need to run is a stack.
 possibility is covered by the `match`, adding further variants to the `enum`
 in the future will prompt a compilation failure, rather than runtime panic.
 Second, it makes cost explicit. In general, the only safe way to have a
-non-exhaustive match would be to panic the task if nothing is matched, though
+non-exhaustive match would be to panic the thread if nothing is matched, though
 it could fall through if the type of the `match` expression is `()`. This sort
 of hidden cost and special casing is against the language's philosophy. It's
 easy to ignore certain cases by using the `_` wildcard: