Examples for ergm-terms to aid understanding

[martinamorris]

Chad developed a helpful example for the dsp(OSP) term . It'd be a pity if that stayed buried in the git repo.

The structure of the help page for ergm-terms doesn't allow for the standard examples at the bottom. We should find another way to add this. Perhaps in the terms vignette?

[sgoodreau]

+1

[krivit]

I think a vignette would be a good idea. I've migrated this to the 3.11 milestone (from the abortive Sunbelt 2020), but the current writeup by @chad-klumb will probably need to be expanded.

[krivit]

The natural place for this will be in term-specific file once #274 is in place.