Improve channel config parameter consistency

[tnull]

It's kind of confusing that we have different lower bounds for channel parameters, such as capacity.

For example, while ChannelHandshakeLimits::min_funding_satoshis defaults to 0 and hence indicates that there is no lower limit, ChannelHandshakeConfig::their_channel_reserve_proportional_millionths effectively sets a 1000sat lower-bound on the channel capacity.

We should improve the consistency of these parameters as well as their documentation.

[TheBlueMatt]

Doesn't need to be release-blocking.

[TheBlueMatt]

Its dumb this has taken this long, but can wait for 117.