Add SSH_DOMAIN.

Ref: go-gitea/gitea#552 (comment)

Author: appleboy

content/doc/advanced/config-cheat-sheet.en-us.md

@@ -64,6 +64,7 @@ Any configuration option that is marked by :exclamation: means that you should k
 - `HTTP_PORT`: HTTP listen port.
 - `DISABLE_SSH`: Disables SSH feature when it's not available.
 - `START_SSH_SERVER`: Starts built-in SSH server when enabled.
+- `SSH_DOMAIN`: Domain name of your ssh server.
 - `SSH_PORT`: SSH port displayed in clone URL, in case yours is not `22`.
 - `SSH_LISTEN_PORT`: Port for the built-in SSH server. Defaults to `SSH_PORT`.
 - `OFFLINE_MODE`: Disables use of CDN for static files and Gravatar for profile pictures.