Missing documentation

[marcusdavidwebb]

There is no documentation for the Lift function. It is mentioned in a sentence and a code comment here, but otherwise there is no description of how the Lift function generates the Tau polynomial. Such a description would be helpful for students learning to use Dedalus. I would contribute to the docs myself, but I don't think I fully understand how Lift works in all cases.

[kburns]

Thanks for the suggestion, I'll try to improve this shortly!