Update default dust exposure limit and the documentation thereof

Update default dust exposure limit and the documentation thereof

Now that we're including excess counterparty commitment transaction
fees in our dust calculation, we need to update the docs
accordingly. We do so here, describing some of the considerations
and risks that come with the new changes.

We also take this opportunity to double the default value, as users
have regularly complained that non-anchor channels fail to send
HTLCs with the default settings with some feerates.

Fixes #2922