[Skip-changelog] Improve documentation regarding ARDUINO_BOARD_MANAGER_ADDITIONAL_URLS (arduino#2136)

* Print error message if `board_manager.additional_urls` contains comma separated urls

* Improve documentation regarding the usage of environment variables

Author: MatteoPologruto

Diff for: docs/configuration.md

@@ -76,6 +76,8 @@ variable sets the `directories.user` configuration option.
 On Linux or macOS, you can use the [`export` command][export command] to set environment variables. On Windows cmd, you
 can use the [`set` command][set command].
 
+`ARDUINO_BOARD_MANAGER_ADDITIONAL_URLS` environment variables can be a list of space-separated URLs.
+
 #### Example
 
 Setting an additional Boards Manager URL using the `ARDUINO_BOARD_MANAGER_ADDITIONAL_URLS` environment variable:
@@ -84,6 +86,12 @@ Setting an additional Boards Manager URL using the `ARDUINO_BOARD_MANAGER_ADDITI
 $ export ARDUINO_BOARD_MANAGER_ADDITIONAL_URLS=https://downloads.arduino.cc/packages/package_staging_index.json
 ```
 
+Setting multiple additional Boards Manager URLs using the `ARDUINO_BOARD_MANAGER_ADDITIONAL_URLS` environment variable:
+
+```sh
+$ export ARDUINO_BOARD_MANAGER_ADDITIONAL_URLS="https://downloads.arduino.cc/packages/package_staging_index.json https://downloads.arduino.cc/packages/package_mbed_index.json"
+```
+
 ### Configuration file
 
 [`arduino-cli config init`][arduino-cli config init] creates or updates a configuration file with the current

Diff for: internal/cli/config/init.go

@@ -17,6 +17,7 @@ package config
 
 import (
 	"os"
+	"strings"
 
 	"github.com/arduino/arduino-cli/configuration"
 	"github.com/arduino/arduino-cli/internal/cli/arguments"
@@ -97,6 +98,13 @@ func runInitCommand(cmd *cobra.Command, args []string) {
 	configuration.SetDefaults(newSettings)
 	configuration.BindFlags(cmd, newSettings)
 
+	for _, url := range newSettings.GetStringSlice("board_manager.additional_urls") {
+		if strings.Contains(url, ",") {
+			feedback.Fatal(tr("Urls cannot contain commas. Separate multiple urls exported as env var with a space:\n%s", url),
+				feedback.ErrGeneric)
+		}
+	}
+
 	if err := newSettings.WriteConfigAs(configFileAbsPath.String()); err != nil {
 		feedback.Fatal(tr("Cannot create config file: %v", err), feedback.ErrGeneric)
 	}