Document GList

I'm sure this leaves a lot to be desired (my understanding isn't very deep), but hopefully it's a start.

can you mention the gc characteristics -- there are 3 options when using an LList:
1. naked Ptr{_GSList{T}} (no gc)
2. GLib.GList (frees the list when the returned reference is gc'd)
2. glist_iter (maintains a reference to the list in the iter state to forestall gc)