fix(core): cut user-facing errors and warnings to one line

docs/dev/rest-api/authentication.md

@@ -42,7 +42,7 @@ This value becomes the one and only API key that the server accepts. The server
     If you start `unshackle serve` with no `api_secret` configured, and you have *not* passed `--no-key`, the command fails immediately with:
 
     ```
-    API secret key is not configured. Please add 'api_secret' to the 'serve' section in your config.
+    No 'api_secret' in the 'serve' section of your config
     ```
 
     Choose a long, unpredictable value. The API key is the only thing standing between the internet and your CDM and download endpoints.

docs/dev/rest-api/remote-sessions.md

@@ -668,7 +668,7 @@ remote session API and a good template for any client.
 
 ## Troubleshooting
 
-!!! example "\"Could not connect to remote server ... Is it running?\""
+!!! example "\"Could not connect to remote server at ...\""
     The client could not reach the URL. Make sure that `unshackle serve` operates on
     the server, the `url`/port in `remote_services` are correct (default port `8786`),
     and any firewall or reverse proxy allows the connection.

docs/guide/proxies-and-vpn.md

@@ -559,8 +559,7 @@ in unshackle itself.
 
 !!! warning "Docker is required"
     Gluetun needs a working [Docker](https://docs.docker.com/get-docker/) installation on
-    your `PATH`. If Docker is not found, the proxy provider fails to load with an error linking
-    to the install docs. The container also needs the `NET_ADMIN` capability and access
+    your `PATH`. If Docker is not found, the proxy provider fails to load. The container also needs the `NET_ADMIN` capability and access
     to `/dev/net/tun` to bring up the tunnel. unshackle requests both automatically.
 
 ### Query format
@@ -643,6 +642,7 @@ proxy_providers:
     - `surfshark`, `mullvad`, and `ivpn` also require `addresses`.
     - `windscribe` additionally requires **both** `addresses` and `preshared_key`
       (`preshared_key` may be an empty string, but the config key must be present).
+      Get both from the [Windscribe WireGuard config generator](https://windscribe.com/getconfig/wireguard).
     - NordVPN and Proton VPN need only `private_key`.
 
 === "OpenVPN"