message: Update publish field doc to explain new `cargo publish` behavior
diff: @@ -406,6 +406,9 @@ allowed to be published to.
  publish = ["some-registry-name"]
  ```
  +If publish array contains a single registry, `cargo publish` command will use
  +it when `--registry` flag is not specified.
  +
  <a id="the-metadata-table-optional"></a>
  #### The `metadata` table
  