GaussianProcess docstring would benefit from enhancements

There is an inconsistency between the celerite2 Getting Started tutorial and the Exoplanet tutorials in the use of `GaussianProcess` and `compute`. Specifically, in the celerite2 tutorial `compute` is called explicitly but in the Exoplanet tutorials that include celerite2, `compute` is implicitly called through supplying the `t` keyword.

I don't think this is a particular problem, but it would be good if the main `GaussianProcess` docstring stated that if `t` is supplied then the compute method is automatically run.

In addition, the `theano.GaussianProcess` class doesn't have a docstring so this would benefit from having the same docstring as the main `GaussianProcess` class, also with the addition of the note about compute.