Revert "remove reference to incomplete crates.io feature from docs"

This reverts commit 1735917.

As discussed in rust-lang#13660, my stance on removing the docs was if there
wasn't a path forward.
There is work progressing on the docs.rs side which will unblock the
crates.io side.

If the concern for removing the docs is for new crates, then no harm in
removing it for now but also little benefit.
If its to get existing crates to change, I don't think that ecosystem
churn is worth it to try to get everyone to add the link and then remove
it again.

src/doc/src/reference/manifest.md

@@ -209,14 +209,18 @@ description = "A short description of my package"
 ### The `documentation` field
 
 The `documentation` field specifies a URL to a website hosting the crate's
-documentation.
+documentation. If no URL is specified in the manifest file, [crates.io] will
+automatically link your crate to the corresponding [docs.rs] page when the
+documentation has been built and is available (see [docs.rs queue]).
 
 ```toml
 [package]
 # ...
 documentation = "https://docs.rs/bitflags"
 ```
 
+[docs.rs queue]: https://docs.rs/releases/queue
+
 ### The `readme` field
 
 The `readme` field should be the path to a file in the package root (relative