Document WhiteKernel under "kernels"
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.

migueldvb/george,jbernhard/george,migueldvb/george,karenyyng/george,jbernhard/george,andres-jordan/george,karenyyng/george,migueldvb/george,andres-jordan/george,dfm/george,andres-jordan/george,jbernhard/george,karenyyng/george,dfm/george