[TiddlyWebWiki] document server.* fields

[jayfresh]

I think it would be valueable to document how the server.* fields are set on a tiddler. For example, server.host comes from this bit of tiddlywebconfig.py:

    'server_host': {
        'scheme': 'http',
        'host': 'ilga.peermore.com',
        'port': '80'
    }