Rollup merge of #134241 - liigo:patch-16, r=dtolnay

more concrete source url of std docs [V2]

r? jhpratt
since you have reivewed #134193

> If someone is looking to contribute, they will want the repository as a whole, not the lib.rs for std.

Now the repository url is reserved, I just add another concrete url as an example, to help people finding target page more quickly&easily.

library/std/src/lib.rs

@@ -89,7 +89,7 @@
 //! Check out the Rust contribution guidelines [here](
 //! https://rustc-dev-guide.rust-lang.org/contributing.html#writing-documentation).
 //! The source for this documentation can be found on
-//! [GitHub](https://github.com/rust-lang/rust).
+//! [GitHub](https://github.com/rust-lang/rust) in the 'library/std/' directory.
 //! To contribute changes, make sure you read the guidelines first, then submit
 //! pull-requests for your suggested changes.
 //!