Update HildonDateSelector documentation

Update HildonDateSelector documentation

* hildon/hildon-date-selector.c
(hildon_date_selector_select_current_date)
(hildon_date_selector_get_date)
(hildon_date_selector_select_day):
Update the documentation: uses always the range (1-31), as it is obvious
that the final one will depend on the month and the year, but globally
the range is this, in a similar GtkCalendar approach
(hildon_date_selector_get_date):
Update the documentation: add NULL as a valid value for the day, month
and year locations

Fix some details, after talk with David King on bug MB#4698