doc: Added fundchannel man page.

[wythe]

This describes fundchannel as it works today (as I understand it). #1360

It definitely could use better error codes instead of -1 for everything.

Is there a better way to tell if the channel is ready than the one I described?

[ZmnSCPxj]

Some nits, also request to change formatting slightly.

[ZmnSCPxj]

Suggest adding newlines at end of sentence. That way, even if an earlier sentence is later amended, only that sentence needs to be reformatted and later sentences in the same paragraph need not be affected. E.g.

The *fundchannel* RPC command opens a payment channel with a peer by commiting
a funding transaction to the blockchain as defined in BOLT #2.
Once the transaction is confirmed, normal channel operations may begin.
Readiness is indicated by *listfunds* reporting a `short_channel_id` for
the channel.

Then later if we amend it:

The *fundchannel* RPC is a totally radical command that opens a really cool payment
channel with a peer by commiting a funding transaction to the blockchain as defined
in the outrageously awesome BOLT #2.
Once the transaction is confirmed, normal channel operations may begin.
Readiness is indicated by *listfunds* reporting a `short_channel_id` for
the channel.

[ZmnSCPxj]

listchannels, despite its name, lists all channels known in the public network, i.e. the entire routemap. Local channels are listed in listpeers.

[ZmnSCPxj]

Yes, we should probably start an effort at some point to give unique error codes for some of the more complex commands. Since it is a JSON-RPC change this will probably have to be done pre-0.6 release too. This is fine for now though; we can update the docs later when we finally have unique error codes.

[wythe]

Yeah that's what I was thinking.

[cdecker]

Should probably also mention that an existing connection is required, and that fundchannel does not attempt to open a connection by itself.

covered

[rustyrussell]

Ack aa0de20