node: open module-specific node documentation (#5572)

plugins/node/node.plugin.zsh

 # Open the node api for your current version to the optional section.
 # TODO: Make the section part easier to use.
 function node-docs {
-  open_command "http://nodejs.org/docs/$(node --version)/api/all.html#all_$1"
+  local section=${1:-all}
+  open_command "http://nodejs.org/docs/$(node --version)/api/$section.html"
 }