mcmc_trace allows highlight but gives a warning

[toxtet]

this is a very minor issue: mcmc_trace allows user to highlight one chain but throws a warning.

bayesplot:::.mcmc_trace allows user to highlight one chain
mcmc_trace does likewise, throws a warning but produces the desired output anyway
?mcmc_trace does not mention the highlight option
mcmc_trace_highlight shows points but not lines

[jgabry]

Thanks, good catch! The original intent was for highlighting to be done only with mcmc_trace_highlight(), but since it actually works for mcmc_trace() too we should officially support it. I just opened a PR.