docs: improve docs of assortativity (#1151) #2609

Job	Run time
Build source R package	5m 15s
Sanitizer	23m 58s
Coverage	5m 54s
Check windows-latest (release)	19m 22s
Check macOS-latest (release)	10m 49s
Check ubuntu-20.04 (release)	6m 21s
Check ubuntu-20.04 (devel)	6m 13s
Check ubuntu-20.04 (oldrel-1)	5m 52s
Check ubuntu-20.04 (oldrel-2)	5m 48s
Check ubuntu-20.04 (oldrel-3)	7m 24s
Check ubuntu-20.04 (oldrel-4)	5m 32s
	1h 42m 28s

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