rlocusplot inconsistent docstring

[agerlach]

1. K is a kwarg
2. OrdinaryDiffEq is always used

ControlSystems.jl/src/pid_design.jl

Lines 206 to 216 in 076615b

	"""
	rlocusplot(P::LTISystem, K)
	Computes and plots the root locus of the SISO LTISystem P with
	a negative feedback loop and feedback gains `K`, if `K` is not provided,
	range(1e-6,stop=500,length=10000) is used.
	If `OrdinaryDiffEq.jl` is installed and loaded by the user (`using OrdinaryDiffEq`), `rlocusplot` will use an adaptive step-size algorithm to
	select values of `K`. A scalar `Kmax` can then be given as second argument.
	"""
	rlocusplot
	@recipe function rlocusplot(p::Rlocusplot; K=500)