Add version note in `Confirm` docs

Add version note in `Confirm` docs

While now `ChannelManager` will only return previously confirmed
transactions, we can't ensure the same for `ChainMonitor`, as we need to
maintain backwards compatibility with version prior to 0.0.113, at which
we started tracking the block hash in `ChannelMonitor`s. We therefore
add a note to the docs stating that users need to track confirmations on
their own for channels created prior to 0.0.113.