Add note on plot_pair to plot_kde(InferenceData) ValueError

[StanczakDominik]

This small text fix adds a reference to plot_pair, rather than just to plot_posterior, when
using plot_kde on InferenceData. This prevents people from trying and failing to use plot_posterior to plot 2D KDEs.

Shoutout and thanks to @OriolAbril, who helped me with finding that function on Gitter :)

Description

Checklist

• Follows official PR format
• Code style correct (follows pylint and black guidelines)
• Changes are listed in changelog

[OriolAbril]

LGTM, can be merged after updating changelog

[OriolAbril]

Thanks for the pull request!

[codecov]

Codecov Report

Merging #1218 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1218   +/-   ##
=======================================
  Coverage   93.14%   93.14%           
=======================================
  Files          94       94           
  Lines        9497     9497           
=======================================
  Hits         8846     8846           
  Misses        651      651           

Impacted Files	Coverage Δ
arviz/plots/kdeplot.py	100.00% <100.00%> (ø)

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