nixos/mautrix-telegram: add documentation for setting arbitrary secrets

The change that allows setting the secrets by an environment file had
the undocumented side effect of allowing to set any key by the environment
file (Related PR: mautrix/telegram#332).

This is required to e.g. set `bridge.login_shared_secret`, which was not
documented before.

GitHub: closes #103347

nixos/modules/services/matrix/mautrix-telegram.nix

@@ -97,12 +97,18 @@ in {
         default = null;
         description = lib.mdDoc ''
           File containing environment variables to be passed to the mautrix-telegram service,
-          in which secret tokens can be specified securely by defining values for
+          in which secret tokens can be specified securely by defining values for e.g.
           `MAUTRIX_TELEGRAM_APPSERVICE_AS_TOKEN`,
           `MAUTRIX_TELEGRAM_APPSERVICE_HS_TOKEN`,
           `MAUTRIX_TELEGRAM_TELEGRAM_API_ID`,
           `MAUTRIX_TELEGRAM_TELEGRAM_API_HASH` and optionally
           `MAUTRIX_TELEGRAM_TELEGRAM_BOT_TOKEN`.
+
+          These environment variables can also be used to set other options by
+          replacing hierachy levels by `.`, converting the name to uppercase
+          and prepending `MAUTRIX_TELEGRAM_`.
+          For example, the first value above maps to
+          {option}`settings.appservice.as_token`.
         '';
       };