docs: Cleanup PyGIBaseInfo repr

Remove angle brackets and pointer address from PyGIBaseInfo.__repr__. This
cleans up documentation and makes it easier to read.
docs: List constructors in object and struct doc strings

Add type dispatching to gi.docstring documentation generator for info types
of StructInfo and ObjectInfo. Add lazy doc string generation to
Object and Struct meta classes by using a property for __doc__. This lists
available constructors immediately in all GObject.Object and Struct docs.
ipython example:

>>> Gtk.Button?
:Constructors:
    Button(**properties)
    new()
    new_from_icon_name(icon_name:str, size:int)
    new_from_stock(stock_id:str)
    new_with_label(label:str)
    new_with_mnemonic(label:str)

https://bugzilla.gnome.org/show_bug.cgi?id=708060