org-datetree.el: Be more restrictive when finding a year datetree

org-datetree.el: Be more restrictive when finding a year datetree

* org-datetree.el (org-datetree-find-month-create)
(org-datetree-find-day-create): Add a docstring.
(org-datetree-find-year-create): Only match headlines with a
year or a year and one or more tags.

Thanks to Suvayu Ali for reporting this.