docs: improve docs of assortativity

[maelle]

Fix #1132

Based on https://igraph.org/c/doc/igraph-Structural.html#igraph_assortativity

@szhorvat is this correct?

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[szhorvat]

I'll need to have a more thorough read later, but the documentation should be read carefully and updated as a whole. We need to make sure that the description / formulas for assortativity match the updated included in the C core docs.

What happened here (in the C library) was that a more general version of assortativity for implemented, and the docs were significantly expanded.

[szhorvat]

I'm sorry, this was a very sloppy review by me, but with this the docs should at least be accurate and complete. We can fix them up when we have the time, after 2.0 ...

[maelle]

It didn't seem sloppy! I've made changes.

[maelle]

@szhorvat could you please re-review this? 🙏

[szhorvat]

I think this should be merged now, as it is all correct. I'll try to make improvements later. Thank you Maëlle!