Document WhiteKernel under "kernels"

Although it's equivalent to setting `yerr` in the `GP.compute` method,
it's good to know that `WhiteKernel` exists, e.g., so you can set/optimize
its parameter via the `Kernel` interface.
allowing mean-only prediction
Merge branch 'document-WhiteKernel' of https://github.com/shoyer/george