TreeModel: Improve docs of foreach*() functions
TreeModel: Improve docs of foreach*() functions

• Fix example slots to have the correct bool return type; they were void
• Don’t use example as 1st paragraph as it ends with ;. which looks odd
• Open by explaining what the typedef is for, with link to its foreach()
• For each foreach*(), name and link to its type of slot in the comment

GNOME/gtkmm,GNOME/gtkmm,GNOME/gtkmm,GNOME/gtkmm