Mention metric in the docstring of RationalKernel (#383)

Author: devmotion

src/basekernels/rational.jl

@@ -5,11 +5,11 @@ Rational kernel with shape parameter `α` and given `metric`.
 
 # Definition
 
-For inputs ``x, x'``, the rational kernel with shape parameter
-``\\alpha > 0`` is defined as
+For inputs ``x, x'`` and metric ``d(\\cdot, \\cdot)``, the rational kernel with shape parameter ``\\alpha > 0`` is defined as
 ```math
-k(x, x'; \\alpha) = \\bigg(1 + \\frac{\\|x - x'\\|}{\\alpha}\\bigg)^{-\\alpha}.
+k(x, x'; \\alpha) = \\bigg(1 + \\frac{d(x, x')}{\\alpha}\\bigg)^{-\\alpha}.
 ```
+By default, ``d`` is the Euclidean metric ``d(x, x') = \\|x - x'\\|_2``.
 
 The [`ExponentialKernel`](@ref) is recovered in the limit as ``\\alpha \\to \\infty``.