roxygen2 warnings

[maelle]

I'm working on #2634 so parking this here for later

> document()
ℹ Updating igraph documentation
ℹ Loading igraph
✖ [cohesive.blocks.R:219](vscode-file://vscode-app/usr/share/positron/resources/app/out/vs/code/electron-browser/workbench/workbench.html#): @aliases must be only 1 line long, not 2.
ℹ The first line is "hierarchy parent export_pajek plot.cohesiveBlocks"
✖ [layout.R:2815](vscode-file://vscode-app/usr/share/positron/resources/app/out/vs/code/electron-browser/workbench/workbench.html#): @aliases must be only 1 line long, not 2.
ℹ The first line is "igraph.drl.coarsest igraph.drl.default igraph.drl.final"
✖ [sparsedf.R:68](vscode-file://vscode-app/usr/share/positron/resources/app/out/vs/code/electron-browser/workbench/workbench.html#): S3 method `[.igraphSDF` needs @export or @exportS3Method tag.
✖ [sparsedf.R:87](vscode-file://vscode-app/usr/share/positron/resources/app/out/vs/code/electron-browser/workbench/workbench.html#): S3 method `[<-.igraphSDF` needs @export or @exportS3Method tag.
✖ [hrg.R:552](vscode-file://vscode-app/usr/share/positron/resources/app/out/vs/code/electron-browser/workbench/workbench.html#): S3 method `as.dendrogram.igraphHRG` needs @export or @exportS3Method tag.
✖ [sparsedf.R:63](vscode-file://vscode-app/usr/share/positron/resources/app/out/vs/code/electron-browser/workbench/workbench.html#): S3 method `as.data.frame.igraphSDF` needs @export or @exportS3Method
  tag.
``