Clarify how to connect to dev desktop

Users can connect to the dev desktops using their hostnames. The
documentation has been adjusted to include their fully qualified domain
names.

Author: jdno

src/infra/docs/dev-desktop.md

@@ -35,9 +35,13 @@ authentication, and automatically fetch the user’s public keys from GitHub.
 You can connect to the instance with the following command:
 
 ```shell
-ssh <your-username>@<instance>
+ssh <your-username>@<name>.infra.rust-lang.org
 ```
 
+Replace `<name>` with the machine name from the table at the top of the page.
+For example, connect to `dev-desktop-eu-1` using the hostname
+`dev-desktop-eu-1.infra.rust-lang.org`.
+
 If you don’t have a public key on GitHub, read the following guides that explain
 how to create an SSH key and add it to your GitHub account. It might take a few
 minutes after the key has been added before the dev desktops get updated.