nixos/mautrix-telegram: document JSON env var config

          and prepending `MAUTRIX_TELEGRAM_`.

          For example, the first value above maps to

          {option}`settings.appservice.as_token`.

          The environment variable values can be prefixed with `json::` to have

          them be parsed as JSON. For example, `login_shared_secret_map` can be

          set as follows:

          `MAUTRIX_TELEGRAM_BRIDGE_LOGIN_SHARED_SECRET_MAP=json::{"example.com":"secret"}`.

        '';

      };