adds missing box selection to core init docs; re #761
adds missing box selection to core init docs; re #761

KonstantinShemyak/cytoscape.js,soomin-maeng/cytoscape.js,rlugojr/cytoscape.js,KonstantinShemyak/cytoscape.js,rlugojr/cytoscape.js,soomin-maeng/cytoscape.js