nixos/mautrix-telegram: document JSON env var config

This was introduced in mautrix-telegram v0.12.1.

See: https://github.com/mautrix/telegram/issues/584#issuecomment-1250027272
See: https://github.com/mautrix/telegram/releases/tag/v0.12.1

pacien 4cdc08f9 e14bdbb9

Changed files
+5
nixos
modules
services
+5
nixos/modules/services/matrix/mautrix-telegram.nix
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and prepending `MAUTRIX_TELEGRAM_`.
For example, the first value above maps to
{option}`settings.appservice.as_token`.
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The environment variable values can be prefixed with `json::` to have
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them be parsed as JSON. For example, `login_shared_secret_map` can be
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set as follows:
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`MAUTRIX_TELEGRAM_BRIDGE_LOGIN_SHARED_SECRET_MAP=json::{"example.com":"secret"}`.
'';
};