Special features of all file format readers should be documented

[szhorvat]

Currently only the "edgelist" format has documentation on the read_graph docc page, https://r.igraph.org/reference/read_graph.html

Specific documentation should be added for all file format readers, mentioning special arguments that are supported (e.g. directed), or any limitations of the reader. Ref #776.

For example, the NCOL reader has these arguments which should be documented: predef = character(0), names = TRUE, weights = c("auto", "yes", "no"), directed = FALSE

[szhorvat]

I'm hoping that someone else can take this. Refer to the C core docs when needed.

[maelle]

Maybe this should even be a vignette? It sounds important (I wouldn't be able to write it, at least not in the current state of my "igraph knowledge")

[szhorvat]

The first priority is to clearly document all options. The reference pages should be complete. Vignettes are an add-on to that.

[maelle]

noting that that manual page has links to the C docs. Beside, there's a Pajek section and a NCOL section.