add docstring for surv syntax

[jburos]

This is related to PR #60 , which implements surv() syntax to be used in the LHS of the patsy formula for a survival model.

Currently the surv function is a stateful transform generated by patsy.stateful_transform().

It would be nice to add a docstring to this function so that a user can type ?survivalstan.formulas.surv to be reminded of the syntax.

Hopefully this can be done by setting surv.__doc__, but I don't know if this will be supported.