server_default poorly documented

[Mike Bayer]

it only includes text(). illustrate how enough information to support an example like:

tbl = Table("derp", metadata,
    Column("arr", ARRAY(Text),
                server_default=array([literal_column("'foo'"),
                                    literal_column("'bar'"),
                                    literal_column("'baz'")])),
)

that is, expressions, literals, etc. as a task elsewhere we have to figure out if we should drop the literal_column aspect of this.