Watch only wallet

[jholdstock]

When setting up decrediton a toggle is displayed to set up a watch-only wallet. There is no explanation about what this means. It would be good to have a brief summary, and a link to dcrdocs which contains more info.

How to configure a watch only wallet.
What a watch only wallet is good for. When they should/should not be used.
Which decrediton features are available/unavailable for watch only wallets.