docs: fix sample_degseq() example

[maelle]

fix #1289

• renamed variables, added explaining variables for clarity.
• conditionally use withr, to make sure it works.

note that rlang::check_installed() will prompt the user to install withr, which is handy IMHO.

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[szhorvat]

How about just doing something like the following and avoiding withr?

while (! is_graphical(degs <- sample(1:100, 100, replace = TRUE, prob = (1:100)^-2)) ) {}

This keeps sampling until we hit on a graphical degree sequence (which happens with a high probability in this case).

I am not really in favour of the name vl_graph ... The Viger-Latapy method creates a simple and connected undirected graph, but we probably don't want to use a variable that spells all that out. I'd rather stick to g1, g2, g3, etc. and spell out their properties later.

No other comments except that I really hate the current method names and I am hoping that #876 will be fixed soon ...