20.4 Explain difference between generic variance and delegate variance

21.4 generic variance and delegate variance are not the same thing, but are similar. Let's add a note to that effect, with an example.

Locate sections 20.4 and 21.4 in the standard sources and read their existing explanations of generic variance and delegate variance. Add a note explaining that they are similar but distinct, including an example, then verify that the wording fits the surrounding standard text.

Written by the indexing model from the issue text.