Publish docs: Clarify requirements about the state of the index after…

… publish.

src/doc/src/reference/registry-web-api.md

@@ -57,6 +57,10 @@ The publish endpoint is used to publish a new version of a crate. The server
 should validate the crate, make it available for download, and add it to the
 index.
 
+It is not required for the index to be updated before the successful response is sent.
+After a successful response, Cargo will poll the index for a short period of time to identify that the new crate has been added.
+If the crate does not appear in the index after a short period of time, then Cargo will display a warning letting the user know that the new crate is not yet available.
+
 The body of the data sent by Cargo is:
 
 - 32-bit unsigned little-endian integer of the length of JSON data.