Documentation: Clarity for HTTPS setups (#5626)

[https-setup]
- Made it clearer that HTTP redirection is possible
[config-cheat-sheet]
- Clarified the behavihour of the redirection-related config keys

Signed-off-by: Matti Ranta <matti@mdranta.net>

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

@@ -122,9 +122,8 @@ Values containing `#` or `;` must be quoted using `` ` `` or `"""`.
 - `LFS_CONTENT_PATH`: **./data/lfs**: Where to store LFS files.
 - `LFS_JWT_SECRET`: **\<empty\>**: LFS authentication secret, change this a unique string.
 - `LFS_HTTP_AUTH_EXPIRY`: **20m**: LFS authentication validity period in time.Duration, pushes taking longer than this may fail.
-- `REDIRECT_OTHER_PORT`: **false**: If true and `PROTOCOL` is https, redirects http requests
+- `REDIRECT_OTHER_PORT`: **false**: If true and `PROTOCOL` is https, allows redirecting http requests on `PORT_TO_REDIRECT` to the https port Gitea listens on.
-   on another (https) port.
+- `PORT_TO_REDIRECT`: **80**: Port for the http redirection service to listen on. Used when `REDIRECT_OTHER_PORT` is true.
-- `PORT_TO_REDIRECT`: **80**: Port used when `REDIRECT_OTHER_PORT` is true.
 - `ENABLE_LETSENCRYPT`: **false**: If enabled you must set `DOMAIN` to valid internet facing domain (ensure DNS is set and port 80 is accessible by letsencrypt validation server).
    By using Lets Encrypt **you must consent** to their [terms of service](https://letsencrypt.org/documents/LE-SA-v1.2-November-15-2017.pdf).
 - `LETSENCRYPT_ACCEPTTOS`: **false**: This is an explicit check that you accept the terms of service for Let's Encrypt.

docs/content/doc/usage/https-support.md

@@ -30,8 +30,22 @@ HTTP_PORT = 3000
 CERT_FILE = cert.pem
 KEY_FILE = key.pem
 ```
+
 To learn more about the config values, please checkout the [Config Cheat Sheet](../config-cheat-sheet#server).
 
+### Setting-up HTTP redirection
+
+The Gitea server is only able to listen to one port; to redirect HTTP requests to the HTTPS port, you will need to enable the HTTP redirection service:
+
+```ini
+[server]
+REDIRECT_OTHER_PORT = true
+; Port the redirection service should listen on
+PORT_TO_REDIRECT = 3080
+```
+
+If you are using Docker, make sure that this port is configured in your `docker-compose.yml` file.
+
 ## Using Let's Encrypt
 
 [Let's Encrypt](https://letsencrypt.org/) is a Certificate Authority that allows you to automatically request and renew SSL/TLS certificates. In addition to starting Gitea on your configured port, to request HTTPS certificates Gitea will also need to listed on port 80, and will set up an autoredirect to HTTPS for you. Let's Encrypt will need to be able to access Gitea via the Internet to verify your ownership of the domain.