rollup merge of #23736: gmjosack/master

Found a few 404s that seemed like simple fixes:

In footer.inc, certain 404 pages were 404ing on the request to jquery.js and playpen.js. This is easily demonstrated by visiting http://doc.rust-lang.org/foo then http://doc.rust-lang.org/foo/bar. The latter 404s, looking for foo/jquery.js.

The Result docs use old_io Writer as an example. Fix the link to old_io Writer. There's probably an effort to update the example away from a deprecated api but this was a simple fix.

rustc/plugin was pointing at the old guide and it was a broken link anyways (plugin vs plugins). Point at the book instead.

The main page of the API docs referenced c_{str,vec}. Looks like these were deleted in 25d5a3a194. Point at ffi docs instead.

src/libstd/lib.rs

@@ -67,9 +67,8 @@
 //! module encapsulates the platform-specific rules for dealing
 //! with file paths.
 //!
-//! `std` also includes modules for interoperating with the
-//! C language: [`c_str`](c_str/index.html) and
-//! [`c_vec`](c_vec/index.html).
+//! `std` also includes the [`ffi`](ffi/index.html) module for interoperating
+//! with the C language.
 //!
 //! ## Concurrency, I/O, and the runtime
 //!