Overhaul of the multisig document

[johnr365]

Monerodocs.org is a great resource.

The page explaining how multisig works had a couple of small issues:

• It was missing details on how to make payments
• One of the commands had changed for the initial wallet setup

I've added:

• More intro details to make the setup process easier
• Details on how to make payments
• A section on how the mnemonic seeds differ

The overall update should make it easier for someone to get started with multisig.

My style of writing is different to the original author, but hopefully it's still very usable.

One query I have regards a URL I inserted:

• I added monero.fail as a possible resource for finding a stagenet node.

The intention was to be helpful, but this may or may not be desirable.

Feedback welcome.