rollup merge of #22049: caspark/fix-link-to-rustdoc

No point sending people to a page which just says "this is now part of
the Rust book" (that page being http://doc.rust-lang.org/rustdoc.html).

Author: alexcrichton

src/doc/index.md

@@ -59,8 +59,7 @@ tools we have are really nice.
 [Cargo](http://crates.io) is Rust's package manager, and its website contains
 lots of good documentation.
 
-[The `rustdoc` manual](rustdoc.html) contains information about Rust's
-documentation tool.
+[`rustdoc`](book/documentation.html) is used to generate documentation for Rust code.
 
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