Improve explanation of dynamic default values for fields

[nickhoffman]

A lot of people on the mailing list are doing things like this:

field :something, :type => Date, :default => Date.today

when they should be putting Date.today in a Proc or lambda:

field :something, :type => Date, :default => Proc.new {Date.today}

The documentation for this can be improved to be more obvious.