tag: documentation enhancement for novice application developers

Adds some code examples how functions can be used. Adds a hint
to look at GQueue if access to the start and the end of the list
is required.

applying comments from Emmanuele Bassi and adds some more
improvements to clarify how functions should be used.

https://bugzilla.gnome.org/show_bug.cgi?id=683388
GList: Some further documentation and formatting tweaks