docs: Clarify that crates.io doesn't link to docs.rs right away.

Author: ehuss

src/doc/src/reference/manifest.md

@@ -203,14 +203,17 @@ description = "A short description of my package"
 
 The `documentation` field specifies a URL to a website hosting the crate's
 documentation. If no URL is specified in the manifest file, [crates.io] will
-automatically link your crate to the corresponding [docs.rs] page.
+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