[feature]: `bakemacaroon`: improve docs about `--root_key_id` option

[ZZiigguurraatt]

When reading the docs (https://github.com/lightningnetwork/lnd/blob/master/macaroons/README.md, https://github.com/lightningnetwork/lnd/blob/master/docs/macaroons.md), it is not clear to me that

lncli bakemacaroon --root_key_id 123 will automatically create a new root key when there is no root_key defined for 123

Instead I'm [mistakenly] thinking I need to run

lncli create --mac_root_key

[only] when initially creating a wallet to change the root key before I'm able to specify a custom root_key_id with bakemacaroon.

We should make it more clear that a new root key can be automatically created every time we try and bake a new macaroon.